1. Place your hands on a low bar so that your fingers are pointing back towards your body and only your fingers are on the bar. Lean forward and down. Try to push your elbows forward at the same time.
This is also a good forearm stretch.
If you do it one arm at a time, push forward on the back of your elbow with the other hand.
2. Put one hand on a wall or pole with your arm straight out to the side, then rotate your body away from your arm like in the chest stretch. Bend your wrist back to stretch the biceps.
for a warm up u can do anything, just do a weight at about 50% of your 1RM for about 15 reps.