Grande Illusions and Grande Illusions 2 by Tom Savini are two excellent books on special make-up effects. Also, check out the SyFy show Face Off, which is a make-up effects artists competition (think Survivor, but with lots of clay and latex).
Also look around on youtube, which has thousands of how-to vids regarding special make-up effects.
thanks. that was really helpful. i think i'll get those and then get Special Makeup Effects for Stage and Screen: Making and Applying Prosthetics next.. which says it has a new version coming out in January
I've never made a head cast but I've tried most of the slasher FX Savini discusses in his book. Used the wrist-slit gag from Martin to freak out a medic when I was in the Army, have done some prosthetic work (sculpting wounds in clay, casting and painting, applying), made some candy glass. Basic knowledge stuff, but I've never tried anything more difficult than those.
Be it film or video, it's all about how you light the effect. Stark overhead lights will likely make seams in the appliance visible; that's why most movie sets use a dull reflector sheet to soften the light and bounce it back at the subject.
The key thing to remember is that once a person can see the seem -- or the zipper on the back of the monster suit, or a badly blended paint job -- the illusion fails and the viewer is taken out of the movie.