Elbow rotations
This one's easy and ideal for the early stages of warm-up. Just move your elbows steadily around in a circle for a while (picture below) and back again. At least a two minutes each way. Increase the radius of the circles so that you really feel that the muscles are busy. Don't go too fast; about 20-30 circles a minute is about right.
Variation: Top

If you have room, do the same exercise with your arms extended, drawing increasing circles with your fingertips.

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