We're going through a bit of a dry spell these days around here, but earlier in the summer the ground was a perfectly soft substrate for animal tracks. At the end of the street there's a patch of land that is either puddle or desert, but animals always pass through that spot on their way to the neighboring farmer's field. Deer, raccoon, and turkey are what we normally find there, and it has become a daily ritual to take a walk after dinner with our first stop being the stomping grounds.
Over a few trial runs, I think we've just about perfected the casting process. I wrote a full article about it here, complete with detailed instructions.
Do a rain dance, and while it's pouring, go get yourself some Plaster of Paris.