Simple Q&A D&D 3.5 (by RAW) XXI

[DeusMortuusEst]

Q 588

Is it possible to make a trip attempt against a prone opponent?

A588 Yes, but it doesn't do anything, as your opponent is already in the position he'd end up in. Basically you'd waste your action, and in worst case you'd get tripped yourself. Better to wait until your opponent tries to get up and provokes and AoO and trip then.