This happened to me once, when I was at school (was probably about 16..?). I was running round a corner in the crowded school yard, and I slipped and lost my balance on a patch of ice. Ended up on my back, lying on the floor, much to the hilarity of the surrounding students.
But I think I deflected it pretty well. I stayed there, punched the air, humorously swore once, and laughed. Didn't get up for about a minute, when somebody helped me. I got the impression that most of my audience were laughing with me rather than at, and they lost interest after a couple of minutes.
The worst thing you can do is look embarrassed, or try to make it look like nothing happened.