I use an air saw with a fine tooth blade or a dremel tool. A hand saw with a fine blade will work also. I recommend that you mark the piece before cutting, then cut to about 1/16th from the mark. Then use a fine file to bring your cut all the way to the mark. You will end up with a much cleaner finish on your cuts.
If you have a dremel, you can also stuff a "Roto Zip" bit in it.
I usually get a few packs of (Metal) Rotozip bits. Work great for cutting plastic/metal/fiberglass etc..