I know this post is long and it may seem a little convoluted,
but stay with me. We'll get there.


SPEECHES --
HP-1, page 137, Snape spoke of "...the beauty of the softly
simmering cauldron with its shimmering fumes, the delicate
power of liquids that creep through human veins, bewitching
the mind, ensnaring the senses... I can teach you how to bottle
fame, brew glory, even stopper death -- If you aren't as big a bunch
of dunderheads as I usually have to teach." He also mentioned
that "...asphodel and wormwood make a sleeping potion so
powerful it is known as the Drought of Living Death. A bezoar
is a stone taken from the stomach of a goat and it will save you
from most poisons."
HP-6, page 177, "The Dark Arts," said Snape, "are many, varied,
ever-changing, and eternal. Fighting them is like fighting a
many-headed monster which, each time a neck is severed,
sprouts a head even fiercer and cleverer than before. You are
fighting that which is unfixed, mutating, indestructible.”
he said, "with a Loving Caress in his voice."
HP-6, page 608, "Dumbledore's eyes were closed; but for the
strange angle of his arms and legs, he might have been
sleeping."
HP-5, page 614, (Fudge) "Or is there the usual simple explanation
involving a reversal of time, a dead man coming back to life,
and a couple of invisible dementors?"
HP-1, page 298, "...Harry, while you may only have delayed his
return to power, it will merely take someone else who is
prepared to fight what seems a losing battle next time - and if
he is delayed again, and again, why, he may never return to
power."
HP-6, page 645, "...he and Dumbledore had discussed fighting a
losing battle not long thereafter. It was important, Dumbledore
said, to fight, and fight again, and keep fighting, for only then
could evil be kept at bay, though never quite eradicated...

(While the Defense-Against-the-Dark-Arts speech was delivered in
a "loving caress," it contained more-or-less the same sentiment as
Dumbledore's "losing battle" speech.
I don't believe Snape's Defense speech is any more suspicious than
his Potions speech. The last one just reminded me of the first,
which included useful information.)

FIRE --
HP-3, page 1, "Witch burning in the fourteenth century was
completely pointless." The "...burning had no effect what-
soever. The witch or wizard would perform a basic Flame
Freezing Charm and then pretend to shriek with pain while
enjoying a gentle, tickling sensation..."
HP-1, page 294 Quirrell, "hunched in pain, looking at his fingers --
they were blistering before his eyes..." "Master, I cannot hold
him -- my hands -- my hands!"
HP-2, page 207 "...when the bird burst into flames. Harry yelled in
shock and backed away... the bird, meanwhile, had become a
fireball; it gave one loud shriek and next second there was
nothing but a smoldering pile of ash on the floor... 'It's a shame
you had to see him on a burning day... phoenixes... They can
carry immensely heavy loads, their tears have healing powers,
and they make highly faithful pets.'"
HP-5, page 616 "...and Umbridge leapt back from Marietta, waving
her hands in the air as though they had been burned."
HP-5, page 622 "Fawkes...swooped low over him.
Dumbledore...grasped the phoenix's long golden tail. There
was a flash of fire and the pair of them had gone."
HP-6, page 576 "But then, through the darkness, fire erupted:
crimson and gold, a ring of fire that surrounded the rock, so
that the Inferi holding Harry so tightly stumbled and faltered;
they did not dare pass through the flames… his wand was
raised like a torch and from its tip emanated the flames, like a
vast lasso, encircling them all with warmth... the ring of fire
moving with them, around them..."

FALLING --
HP-1, page 189, Snape helped to control Harry's bucking broomstick.
HP-3, page 181, "...when the dementors came onto the Quidditch
field, Dumbledore "...ran onto the field as you fell, waved his
wand, and you sort of slowed down before you hit the ground..."
HP-6, Page 596, "...as Dumbledore was blasted into the air. For a
split second, he seemed to hang suspended beneath the shining
skull, and then he fell slowly backward, like a great rag doll,
over the battlements and out of sight."


Avada Kadavara --
HP-1, page 29, "He couldn't remember being in the car when his
parents died. ...when he strained his memory... a blinding flash
of green light and a burning pain on his forehead."
HP-4, page 4, "...the Riddles all appeared to be in perfect health --
apart from the fact that they were all dead."
HP-4, page 15, "There was a flash of green light, a rushing sound,
and Frank Bryce crumpled. He was dead before he hit the floor."
HP-4, page 216, "There was a flash of blinding green light and a
rushing sound, as though a vast, invisible something was
soaring through the air -- instantaneously the spider rolled over
onto its back, unmarked, but unmistakably dead."
HP-4, page 638, (Kill the spare) "A blast of green light blazed
through Harry's eyelids, and he heard something heavy fall to
the ground beside him... he opened his stinging eyes. Cedric
was lying spread-eagled on the ground beside him. He was
dead. ...Harry stared into Cedric's face, at his open gray eyes,
blank and expressionless as the windows of a deserted house,
at his half-open mouth, which looked slightly surprised."
HP-5, page 721, (McGonagall) "...the red beams collided with her.
For a moment she looked luminous, illuminated by an eerie red
glow, then was lifted right off her fee, landed hard on her back,
and moved no more.
HP-6, Page 596, (on the tower) "A jet of green light shot from the
end of Snape's wand and hit Dumbledore squarely in the chest.
Harry's scream of horror never left him; silent and unmoving,
he was forced to watch as Dumbledore was blasted into the air.
For a split second, he seemed to hang suspended beneath the
shining skull, and then he fell slowly backward, like a great rag
doll, over the battlements and out of sight.
HP-6, page 599, "'Impedimentia!' yelled Harry. His jinx hit
Amycus in the chest: He gave a pig like squeal of pain, was
lifted off his feet and slammed into the opposite wall, slid
down it, and fell out of sight.."
(Snape's curse on the tower doesn't match the other times we've
seen the Avada Kadavera curse.)


MIND GAMES, Occlumency, Nonverbal spells --
HP-5, page 615 "...Harry heard a rustle behind him and rather
thought Kingsley whispered something. He could have sworn
too that he felt something brush against his side, a gentle
something like a draft or bird wings, but looking down he saw
nothing there."
HP-5, page 616 "I don't think you understood the question, did you,
dear? I'm asking whether you've been going to these meetings
for the past six months? You have, haven't you?' Marietta
shook her head." Later, she was "still clutching her robe up to
her oddly blank eyes, staring straight ahead of her."
HP-6, page 178, With a nonverbal spell, "Your adversary as no
warning about what kind of magic you are about to perform,"
said Hermione, "which gives you a split-second advantage."
HP-6, page 592, "No, Draco," said Dumbledore quietly. "It is my
mercy, and not yours, that matters now."
HP-6, page 603, Teaching right to the end: "Blocked again and
again and again until you learn to keep your mouth shut and
your mind closed, Potter!" sneered Snape, deflecting the curse
once more.
HP-5, page 531, Snape said, "The Dark Lord... almost always
knows when somebody is lying to him. Only those skilled at
Occlumency are able to shut down those feelings and
memories that contradict the lie, and so utter falsehoods in his
presence without detection."
(I've felt for a long time that Snape was faking it. The childish
hostility between James and Snape when they were kids doesn't
add up. He should have grown up and moved on by now.
I'm sure that the point of Snape's carefully cultivated hatred of
Harry is for a purpose. Dumbledore tolerates it because it is
a part of Snape's way of fooling Voldemort. Snape is able to
"shut down those feelings and memories that contradict the
lie," but that is not enough. Eventually, Voldemort would
notice a blank, stone wall where a person with emotions should
be. Snape needs to fill that blank space with something
credible. This is what he put in the Pensieve during
Occlumency lessons. The one Harry saw was just a cover,
which Snape intended Harry to see.)


Misc. --
HP-1, page 283, "Yes..." said Ron softly, it's the only way. I've got
to be taken... That's chess!" snapped Ron. "You've got to
make some sacrifices!"
HP-5, page 533, "Snape... deposited the silvery substance into the
stone basin, then, without offering any explanation of his
behavior, he picked up the Pensieve carefully, removed it to
a shelf out of their way..."
HP-6, page 591, "He cannot kill you if you are already dead.
Come over to the right side, Draco, and we can hide you more
completely than you can possibly imagine. What is more, I can
send members of the Order to your mother tonight to hide her
likewwise. Nobody would be surprised that you had died in
your attempt to kill me -- forgive me, but Lord Voldemort
probably expects it. Nor would the Death Eaters be surprised
that we had captured and killed your mother -- it is what they
would do themselves, after all. Your father is safe at the
moment in Azkaban... When the time comes, we can protect
him too. Come over to the right side, Draco... you are not a
killer..."
HP-6, page 608, "He had known there was no hope from the
moment that the full body-bind curse Dumbledore had placed
upon him lifted, known that it could have happened only
because its caster was dead..."
HP-6, page 643, Hagrid carried him, "...wrapped in a purple velvet
spangled with golden stars, was what Harry knew to be
Dumbledore's body... Hagrid seemed to have placed the body
carefully upon the table.


TRICKY MOVES --
HP-1, page 213, Dumbledore said, "I don't need a cloak to become
invisible."
HP-5, page 838, "...and I have watched you more closely than you
can have imagined."
HP-5, page 622, "Dumbledore...grasped the phoenix's long golden
tail. There was a flash of fire and the pair of them had gone."
Later, as the door swing closed behind them, "Harry heard
Phineas Nigellus's voice, 'You know, Minister, I disagree with
Dumbledore on many counts...but you cannot deny he's got style....'"
HP-5, page 818, He pointed his wand at it and muttered, "portus"...
"Take this Port Key, Harry."

=============================

HOW does Dumbledore move around without being seen?
Even he can't usually disapperate inside Hogwarts. Except...
in HP-6, page 382, "As you may know, it is usually impossible
to apparate or disapperate within Hogwarts. The headmaster
has lifted this enchantment, purely within the Great Hall, for
one hour, so as to enable you to practice. May I emphasize that
you will not be able to Apperate outside the walls of this Hall,
and that you would be unwise to try."

I'm sure Dumbledore has many methods, one of which is the Time Turner.

FOR EXAMPLE:
In HP-3, Hermone's need for a time-turner was too lame. It had to
be an excuse to introduce the gadget, for a purpose that JKR
has not yet revealed. I suspect that Dumbledore uses it regularly.
HP-5, page 618, "Well, the game is up" he said, simply. "Would
you like a written confession from me, Cornelius -- or would a
statement before these witnesses suffice?" Later he instructed,
"...you must act as though no time has passed, as though they
were merely knocked to the ground; they will not remember."
HP-5, page 622, "Dumbledore...grasped the phoenix's long golden
tail. There was a flash of fire and the pair of them had gone."
Later, as the door swing closed behind them, "Harry heard
Phineas Nigellus's voice, 'You know, Minister, I disagree with
Dumbledore on many counts...but you cannot deny he's got
style....'"

Where did he go? Back in time a few hours, (1) to give his
previous self a few helpful clues (Like, "You're in for an
interesting evening. I won't spoil it for you, but..." and
"Play dumb, wait until you see a list of names on a sheet of
parchment, then confess to everything) and (2) to tell
Shacklebolt to be prepared to use a memory charm, "on my
signal."

=============================

SO, WHAT HAPPENED THAT NIGHT ON THE TOWER?

First of all, yes, I believe Dumbledore and Snape had a general
plan. While they could not have worked the details out in
advance, Dumbledore expected something like this and,
like Ron did so long ago, said, "It's the only way. I've got
to be taken."

Remember Dumbledore high in the air -- freeze that image.
We'll come back to it in a moment.

So, here's my version of events:

As many others have suggested, Snape faked the Avada Kadavara
curse. What he really used was some other, non-verbal spell.

Snape left the tower, ran through the battle scene (pg 621), and said,
"It's over." Once out of sight, he ditched Draco for a moment
near a conveniently located, deserted room and turned his time
back a few minutes.

He went outside, below the tower, and caught Dumbledore as he fell.

Then, Snape used the time-turner to go back a few hours, this time
with Dumbledore. The full body-bind curse Dumbledore had
placed on Harry lifted, not because he was dead, but because
he vanished back in time.

Snape and Dumbledore slipped away to rest and regroup, to discuss
and make plans.
Snape administered a bezoar potion and/or performed whatever
immediate healing magic was necessary, and patched up a few
wounds of his own.
Dumbledore had a chance here to look at the locket and RAB's
note, and planned to place it so that Harry would find it.
He took the time-turner back from Snape, who now had no further
use for it. Dumbledore would use it at his own "funeral."
They could have run a few other errands here, if you like.
Perhaps they left a note to McGonagall in the headmaster's
office that she won't find for days or weeks.
Perhaps Snape sent some message to his previous self so that
he could collect some potions and be prepared when Flitwick
brought him the news.
Dumbledore took some wormwood/asphodel potion (the Drought
of Living Death) and
Snape carefully planted Dumbledore at the foot of the tower --
"Dumbledore's eyes were closed; but for the strange angle of
his arms and legs, he might have been sleeping."
The bezoar probably didn't do the whole job, and Fawkes had his
work cut out for him this night.

When the time caught up with him, Snape slipped back into that
conveniently located, deserted room to rejoined Draco and
proceeded out through the gate.

At the funeral, (HP-6, page 645) "Bright, white flames had erupted
around Dumbledore's body and the table upon which it lay;
Higher and higher they rose, obscuring the body. White
smoke spiraled into the air and made strange shapes: Harry
thought, for one heart-stoping moment, that he saw a phoenix
fly joyfully into the blue, but next second the fire had
vanished." (What can I say? The guy liked his fire and his phoenix.)

HP-6, page 626, The new portrait of Dumbledore "was slumbering
in a golden frame over the desk, his half-moon spectacles
perched upon his crooked nose, looking peaceful and
untroubled," because the real Dumbledore WAS somewhere,
sleeping peacefully and untroubled.

The day had, after all, been long and exhausting.