Watched the first series again yesterday, and the second again today, so I have new thoughts on the cliffhanger! I've also gone back through the thread to make sure I'm not rehashing old ideas.
Obvious ones out of the way first: Molly is in on it, probably running the morgue side of things, making sure he is announced dead; Sherlock's homeless network also helps (bicycle man, for example).
I think there are two main components to his survival - the Baskerville drug from episode two, and the drug Adler used on him in episode one (she jabbed him with a needle that appeared to contain a sedative of sorts).
When John gets knocked over by bicycle man, he hits the ground and we hear a high-pitched ringing. As this is John Watson, war veteran, I can't believe a simple knock confused him, so why the ringing? When he was locked in the lab in the second episode, we heard a high-pitched ringing as he began to get scared. I'm going to make an assumption that pumping adrenaline is what sets the Baskerville drug off - something he's definitely going to experience having seen Sherlock jump off a building and then be surprised by bicycle man. And, of course, as subjects become highly suggestible on the drug, Sherlock's speech made John see what was suggested to him. Question is, when did Sherlock administer it?
What of Adler's drug? Sedatives are neat little things that can be used to just calm someone down, numb pain, send a person to sleep, and, if administered in a large enough dose, overwhelm the body entirely. As Sherlock is completely lucid up until hitting the ground, we can probably rule out him self-administering. Luckily, there are plenty of opportunities for a bystander to inject Sherlock as he is laying on the floor, because there are long periods of time that John cannot see him for - the rubbish truck, John being knocked down, and the crowd around the body. My thoughts are that one of Sherlock's helpers sedated him after he hit the ground for two reasons - firstly to numb what has to be excruciating pain (you know, just in case he wasn't unconscious), and secondly to give the impression of death. Sherlock is a man who knows everything, and as such a man can most assuredly calculate how much of a sedative he should be given to almost kill him, but not quite.
As for the fall? Calculation, and sheer luck. There are plenty of documented cases of people surviving long falls, so Sherlock was able to pick a building he could jump from with a good enough chance that he'd survive, whilst still looking fatal.
---
In chronological order:
* Sherlock realises he has to die to beat Moriarty.
* Sherlock works out which building he can survive a fall from.
* Sherlock enlists Molly to help in his plan, along with his homeless network.
* At some point on the day, Sherlock drugs Watson with the Baskerville drug.
* Sherlock meets Moriarty, the scene plays out, and Sherlock jumps, having suggested suicide to John over the phone.
* Sherlock hits the ground, shattering many bones, damaging a lot of tissue, and almost killing himself.
* John, with the drug and adrenaline coursing through his veins, sees what he thinks is a fatal fall, but does so in a state of confusion, partly thanks to bicycle man.
* Whilst John can't see the body (rubbish truck, bicycle man, crowd, etc), Sherlock is injected with a sedative to give the impression of death.
* John gets to Sherlock, tries to take a pulse, but fails and assumes Sherlock is dead.
* Hospital workers rush Sherlock into the hospital.
* Molly works some magic.
Some months later (I can't work out if it's eighteen months from the incident, or eighteen months from meeting Sherlock), John's therapist tells him to say what he wants to say, so John does so at Sherlock's grave. At this point, Sherlock has recovered enough to look healthy and be able to move around outside.
---
It's either that or voodoo.
Though probably something else entirely.