Truth be told, patience. I tried using tape once, but I think I picked a poor quality tape, because the paint managed to work its way under the tape. So, until I find a better product, I've just been using a small brush and a steady hand.
I'm glad everyone's digging the figures, but I'm a little disappointed that no one's commented on my updated Iron Spidey. I worked hard on sculpting his arms and fashioning them so that they are removeable.
Well... I think you coud use masking film, that's effective, at least, it worked with me.