First there should be two ways of getting on one... (they should NEVER ever be sold at some ranch)
1. Climb up a large rock or cliff, and as the cliffracer charges JUMP... with the right skills, practice, and timing, you should be able to land on it...
2. Cast a command creature spell and force it to pick you up...
3. Jump from the Cliffracer your riding to a cliffracer below...
Either way they should not like you being on them and should try to throw you off, or stab you with their tail (of course not under the command spell though)... attacking other things would involve sliding down to the edge of a wing then pulling your %PCWeapon (for attacking creatures to the sides), standing up, then swinging your weapon (for attacking creatures above), and finally grabbing the "fin" and with all your might flip the beast(for attacking creatures below)... each attack should have a huge chance of you falling off... and every single move should have a small chance aswell...