When I was a kid in school, we made face casts by slathering on Vaseline to protect the skin and then use strips that we dipped into some kind of watery glue mix. We layered the strips on the face, waited impatiently for them to dry, and carefully slipped them off. Voila! There are far more sophisticated methods, however ;-)
You can find casting supplies at a good art or hobby shop (in Perth, check out Kirkside Products). If you're a DIY type, Google "how to make a plaster cast of your body" for lots of ideas including YouTube videos. Look at this one to see a method using Plaster of Paris bandages. It's pretty cool.