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I am making a cast of a man sitting down. I am using plaster to assemble the pieces, and when I have finished, I would like to try casting it with silicon. Any tips and tricks on doing this? What about creating “part lines”? Do I need to use shims?

I make large silicone molds in two halves (called “book molds”).
You need to decide where to create the parting line and then add a clay shim all around the parting line. Build a box-form around everything to hold the silicone. Mix your silicone and brush it on creating a comfortable thickness. Allow this to cure overnight. Next make a mother-mold for the silicone section with Hydrocal mixed with hemp or loose strands of fiberglass. You need to create positive-nipples by "pulling" the silicone as it sets. Allow the nipples to protrude through the Hydrocal mother-mold so it locks to the mother mold.
Once Hydrocal sets, flip over the mold, remove the clay shim and clean all of the surfaces. Re-box the mold and brush on the second half of the silicone and again, reinforce this second half with another mother-mold.
The positive nipple from the first half will create a negative in the second half, to keep both halves perfectly aligned.
Happy casting! ~ Joe

You should use Poly-Vinyl Alcohol (PVA) to act as a mold release between the silicone rubber mold and a fiberglass-resin cast. You can check with your supplier as to what types of release agents they have.
When making a piece-mold, be sure to make sections and inserts for all of the undercuts. Make sure they lock into each other, similar to what was described in the last posting.