If your rotors are glazed, you can get a drill-mounted, Surfacing Discs kit that will help you remove the glaze, or if it real bad and/or are warped you may want to have the rotors re-surfaced on a lathe at an automotive place.

My rotors where not too bad and really did not need anything done to them, but I felt like cleaning the surface of them.  I used a drill mounted sanding disc from 3M ( as shown below) and with the rotors removed, cleaned of any of the glazes that was on them.  Made them look just about like new again.  This is also good for use on rotors that you just had turned on the lathe, since it helps remove the directional scratch pattern after being turned.
 
 

The rotor surfacing kit is designed to produce a non-directioal scratch pattern as a filal step after turning brake rotors.  Help eliminate brake squeal and improves stopping power.