Something you may want to try instead of vaseline (or hair gel ) is saran wrap. Once you pull it down over the head over the head just to point before it breaks it is super thin and stuck right to the sruface of the head. You sculpt on top of it and when it dries, the plastic wrap pulls right off of it with no sticking. You get a nice tight fitting hat.
Then I got an idea. Can you use or has anyone tried saran wrap around a sword to sculpt a sheath? I'm going to give it a try when I get a chance this weekend. Thanks Pluv!