Should you decide to use a live person instead, read this link. It looks pretty good and I don't feel like typing out all of the instructions. If it's not good enough then google "diy death mask". Don't do this with someone who is claustrophobic.
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...or still breathing....;-)
Well, my link was instructions for making a "death" mask on someone who is still alive. I think that's acutally called a life mask.
If you don't have a corpse handy, try this body-casting Instructable, or this one, which also involves chocolate*.*And the occasional nipple.
First find your corpse, then using plaster of paris make a mould of their face, in-situ.