Do you threaten while prone? This comes up all the time for one of my groups, someone is tripped, ends up prone, but can still take AoO's and provide the flank for another character. I've had a few battles where once a character is prone they stay that way until they are no longer threatened by an enemy and then stand without drawing an attack of opportunity. I haven't seen anything in the rules disallowing it (and the players will often gladly take the -4 AC/to hit, just so they can get their full attack action).