If you are one of the myriad guitarists that suffer from sore hands due to too much—or in some cases not enough—practice, consider getting a pair of Chinese Healthy Balls and taking matters into your own ...
I used to live on Nob Hill in San Francisco, just a few blocks from Chinatown, and various sizes of these balls were available everywhere. Healthy Balls (a.k.a. Baoding Balls, Chinese Exercise Balls, Chinese Meditation Balls) have been popular in China since the Ming Dynasty (1368-1644), and using them daily is said to improve hand strength and dexterity. In my experience, that is absolutely true—but using them can also help relieve the sorts of hand pain that comes from too much practicing of difficult stretches, wide-voiced chords that require a lot of pressure to execute, and other muscle abusers.
My Healthy Balls are on the large size, and work well for my purposes, though the traditional wisdom is that you should start with smaller balls and work your way up to the larger sizes. Ideally, with smaller balls you should be able to rotate them in your hand with such a smooth and regular motion that they do not come in contact with each other. This is obviously impossible with larger balls, unless you have really big hands.
Here's a rather crude demonstration of how it is done:
Chinese Healthy Balls of all sizes are available on Amazon. Have fun!