Warming up the muscles is:
-A great way to get loose
-Prevent injuries
-Start the rehearsal on a good note
-Discussion time since they are all in one area with few distractions
I personally don't have a defined set of things I do, but I stick to the basics of ankles, calf, arms, core, and upper/lower back. All of those stretches in a circle among each other, and a nice little jog. Depending on how demanding your show or rehearsal is, a good stretch can really make the difference. Just getting limber and having your heart already in that motion keeps you alert and gives you a nice note to start the day on.
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IPT-Bass Five '09
IPT-Bass Four '10
TAMUK Quads '11