Image via Wikipedia
One effective way of doing this is simply to use the brakes frequently, especially if you live in a wet and humid climate. This scrubs off any rust that may be starting to form on the rotors. But this method is only good if you drive frequently. Those who drive more rarely will have to clean the rotors.
Accessing the Brakes
Brake cleaner can be purchased in aerosol cans. After removing the wheel, you may want to remove the brake caliper so the entire rotor can be cleaned. Place a catch pan underneath the brake assembly to prevent cleaner from dripping onto the garage floor. Never use compressed air to clean brake parts, the poisonous asbestos dust on them will take to the air. If rust has begun to accumulate, oil can be used to remove it, provided that this is not used on the surface of the rotor that the brake pads touch. Make sure all the cleaning oil is thoroughly removed from brake parts before you drive the vehicle.
Use Zinc Rotors
To help prevent rusting, replacing the stock rotors with zinc-coated ones is a good idea. Cadmium-coated rotors will achieve the same result, but when cadmium wears off, it may cause environmental problems.
This is one of those relatively simple regular maintenance jobs, but it is important to remember to actually do the job from time to time, especially if the vehicle is not often taken for a spin. You may one day discover that the rusting is far gone, and will be more of a problem to remove. Once again, prevention is better than the cure.