I make 1/12"cale miniatures & would like to create my own plaster castings for architectural trims, etc. Would polymer clay make a good reusable mold? It seems that plasticene would make a fair one-use mold, but I'm thinking multiple elements here (like crown molding motives, etc)

Plastiline is ok for casting plaster but it is not very strong and may warp or deform when pressing in the model to create the mold.
There is a machine shop near me and they sharpen drill bits and saw blades. When finished they dip the tool into a melted wax type substance which hardens and has a natural oil base so no release is needed. I do not know the name of it but it is very common. It melts at low temperature and cools and hardens rapidly. When hard it is perfect for molding as it flexes and will not stick to anything I have found. After use I just put it back into the pot for use again. I'm sure you can find some by checking a machine shop or took sharpening shop.
Jerry K.
www.flastatuary.com

Latex rubber molds are used by most manufacturers for repeated use. Some have used the same molds for over twenty years. Good luck

mini thanks for all the info, I've been away for a while.