Ride along quite fast, and hold your brake on a little bit, it will warm up and if you hold the brake in the right place it will warp itself straight. I used to keep having to do it with mine, really p*sses me off! Let me guess, its a mono trial brake/rotor? Oh yeah, and i know its not as fun as smashing the crap out of it with a hammer, but if its a mono trial, the rotors are so weak you can just bend it back with your hands!