If you're talking about a latex mask it's easy. Scrub it in warm soapy water with a soft yet still some what stiff bristle brush. I've also heard of people using some sort of mild citrus cleaner to help remove the paint as well, but have no experience with it. The main thing you want to be concerned with is removing any loose bits of paint. You don't really need to worry about removing all the paint. Once that's done, coat it in whatever you're using as a primer coat (prosaide, etc.) and go from there. Just curious... what did you use to paint it with?
Brian