Running theories I've seen around the interwebs:
1) Sherlock jumped and landed in the truck that's briefly seen when John turns the corner and sees Sherlock's body on the ground. He survives the fall this way.
1a) The body wasn't really Sherlock; the real Sherlock rides off in the truck after having placed a different body (from Molly's morgue?) on the ground.
1b) The "body" was Sherlock, but he was alive (after landing in the truck). When John gets knocked down by the cyclist, he gets dosed with something to disorient him. Cyclist + some/all onlookers are part of Sherlock's "homeless" network from the original stories. When John reaches to feel Sherlock's pulse, someone takes his hand and pulls it away.
2) Sherlock somehow made the switch on the roof, and the person falling was really Moriarty.
2a) Moriarty made the little girl TERRIFIED of Sherlock somehow, could have worn a lifelike Sherlock mask or something?
In all scenarios, Molly helps by pronouncing him dead.
Personally, I'm partial to the theory that Sherlock survived and prevented John from realizing he wasn't dead.
Also, I wasn't bawling until John was standing at his grave, begging him not to be dead. Then it was all over for me.