One tip I can give you is it's never a bad idea to spend a little extra time making sure all of your layer lines and seams are filled in and smoothed out before painting.
Personally I start with a fairly low grit sandpaper on an orbital sander. Then I start using spot putty on the large imperfections, and filler primer in the rest of the areas to thoroughly smooth out the helmet, sanding with higher and higher grit sand paper every time.