Like manisusmc says - water will warp them.
Also, I don't know whether you have an auto or stick tranny - autos are tougher on brakes than sticks. If you've driven the car hard and done some hard braking, don't sit there with the brake pedal pressed, use the e-brake or chocks if you can. The brake pads will retain heat in that part of the disc where they contact, causing uneven cooling of the rotors - helping them warp.