Below are some stretching exercises to keep you warm and loose during your guitar lesson. Just take 2 minutes before you practice and you can save yourself a lot of trouble in the long run. The can help your guitar technique by keeping you loose and relaxed.
I borrowed these exercises from Aikido but they have worked pretty well for me. With this exercise, grab around your wrist touching your thumb and middle finger if possible. Apply pressure to the backside of your hand with your wrist. Your elbows should naturally gravitate downward. This will strengthen your wrist and stretch the forearm.
Grab the fleshy part of your thumb with your fingers. Your thumb should lie across the backside of your hand pressing on the bug knuckle of your middle finger. Push with your thumb and pull with your fingers. This will stretch the forearm and strengthen the wrist.
Lay your hand out in front of you like you're offering something. Grab your fingers as shown and pull down and back. This will stretch the forearm and the wrist. It will also stretch the palm of the hand. Don't over do it.
This exercise will strengthen and stretch your hands and fingers. This is basically done the way it looks. (as the others) Put your fingers together like you're praying. Press them together and spread them apart at the same time. Again don't over do it and hurt yourself. Just get a good stretch. Below is the side view.
After you do these stretches, do some wrist circles clockwise and counter-clockwise. Then shake them out to get rid of the tension.