Good abilities:
improvements to charging attack and damage
improvement to mobility while charging (jumping, turns, terrain)
combining overruning/trampling with charging
improvements to bullrushing itself
piggybacking attacks onto bullrushing or vice-a-versa.
The best abilities/classes/feats I know of include:
Warforged Juggernaut and Avalanche. Avalanche is from the Players Guide to Arcanis. It is a high level and high power feat excellent for bullrushers. Speak with your DM to see if he'd consider allowing it.