WLM, first thing you need to do is remove anything that is easily removed like boom mics, snaps, etc... Then lightly sand the painted part of the helmet with 400 or 600 grit sand paper or what I like is a 3M product called a Scotch Brite pad. You might find a simular pad near the kitchen sink. Anything that will clean and deglaze the paint will work. Then mask off the areas that you don't want to get paint on like the edge roll and the inside. I forgot to mention that if the surface has gouges they need to be feathered out until you can't feel them with your finger nail. Clean the surface with Acetone or Lacquer thinner. Then get someone that can use and has access to a good quality poly-urethane paint to apply to helmet surface. Three coates should be enough. It sounds easy and it's not hard but the preparation is the secret to getting a good paint job. Take short cuts and it will show up.
I personally like Alumigrip paint. You can paint your helmet with it and once it is cured you can throw it around or bang you head against the doors or windows and it will stay if the surface was clean and applied right.
Barry