I hope I'm not Captain Obvious with this, but I found that you can cancel the POW block with a trick if you time it right. I'm not talking about finding a ramp and doing a trick off it - you can cancel it on straight flat ground. Just time it so you press the trick (if you're using the gamecube controller like me, press the d-pad) at the exact moment the POW block hits you. It should cancel with barely any speed loss.