Just dont paint the rotor, if you get some on the rotor, just wipe it off with brake cleaner. you can avoid buying that expensive caliper paint kit by using Engine paint. Thats all i ever use and works just as well. Mine have been painted since the first week i had my car in Nov '04 and still look like i just painted them. Best way is just take the caliper loose and have it hanging by the brake line, clean it with brake parts cleaner then wipe it down with alcohol. Then paint using a couple coats of the engine paint. Also, it you want to speed things up, you can use a heat gun or hairdryer to dry it.
Masking does work if you pretty much cover everything and have some sort of shield protecting the fenders and such. But just make sure you have enough masking tape, newspapers and probably cardboard to act as a shield (just take not of wind direction when you spray )
But i had my car sitting on jacks for a couple days while i painted mine...although i used a brush kit so i didn't bother to really mask anything off.
i think i'll do this next weekend but i think i'll use the brush kit. no dealing with over spray and taking the caliper on and off and all that. not as much to tape off because of the no spray. figure that's easier.