Unseelie said:
Oh, and don't forget that your AOO for him getting up can, indeed, be another trip attempt which would then allow you to follow it with yet another attack if you have Improved Trip.
Actually, you can't take the AoO for standing up as a trip (or more precisely, it does no good).
In game, the AoO occurs before the action that provokes it resolves. You provoke the AoO by standing up. In order to be able to stand up, you can't currently be standing. (And you aren't standing (but are prone) until the AoO resolves). So, the stander is prone when the AoO occurs. After the AoO occurs, the stand up action resolves and the character is standing. Since the character is already prone at the time of the AoO, tripping him then doesn't change anything.
If you have a PC or PCs that are taking serious advantage of Trip, may I suggest the prone fighting feat from Complete Warrior.
Another good idea is throwing in a mix of non-humanoid foes. A giant, minotaur, ogre, etc is very difficult to trip (and remember--Imp Trip doesn't remove the possibility of being tripped in return). So are dwarves, derro, druergar, etc. Incorporeal creatures can't be tripped. And four (or more) legged creatures are very difficult and risky to trip too. Bears, lions, dogs, hippogriffs, Remorhaz, purple worms, carrion crawlers, etc are very difficult to trip. And creaturest that don't really have a discernable anatomy probably can't be tripped either. Tripping a gibbering mouther or any ooze is pretty much impossible to imagine.
If you insist on throwing primarily humanoid foes at your PCs, you'll find that some abilities (like Imp Trip, Imp disarm, and enchantment spells, etc) are overly powerful and some (like Improved Turning, Extra Turning, and many favored enemy choices) are much less powerful or even useless. That's not necessarily because of a design flaw, however. It's because the game balance assumes a variety of encounters and games that deviate strongly from that will find that the balance shifts as well. (Just like Great Cleave is much more useful in campaigns that reliably feature hordes of enemies and Spring Attack is more useful in campaigns that regularly feature lone, overwhelming foes).