Keep in mind, too, that the AoO from getting up is not so bad - it cannot be used to knock you back down (trip, whatever) because the AoO happens before you get up.
Off-hand, the only way I can think of to keep a prone character prone is to use a readied action to trip him again (or knock him prone - however you do it). Even then, since getting up is a move action, he could just get up again as his second move, though he'd get no attack that round. The the person with the readied action would go first ('cause that's how it works) and could knock you back down again.