Pain is your body's way of telling you you're doing something wrong. If your hands or wrists start to hurt, STOP PLAYING. You may need to address some element of your technique that is causing the strain, or at the very least you need to give your body a rest. You can do permanent damage if you continue to play through pain. Pain in the wrists is often diagnosed as tendonitis, or worse, carpal tunnel syndrome. If you are experiencing pain regularly or if it is severe, see your doctor.

Here is some advice for avoiding injury:
Before you play, stretch out your hands, fingers, and arms. Warm up slowly, take frequent breaks, and stop when it hurts. Also, pay attention to your playing position. Holding your guitar too low is a major contributor to carpal tunnel syndrome. Try to keep your wrist as straight as possible when fretting.

Also observe these general guidelines:

1. Drink plenty of spring water or filtered water. Attempt to drink one 8 oz. glass of water per hour. The tissues of your body are composed mostly of water. Dehydration inhibits the healing process.
2. Avoid caffeinated beverages and alcohol. These will dehydrate you. The stimulating affect of caffeine will also tense your muscles.
3. Before you begin your workday, stretch your muscles, and perform some light exercises to "wake them up." Avoid working with tight, unstretched arms and shoulders. This will lead to further injury.
4. Perform stretching exercises at your workplace at least once per hour. (Provided by a health care provider)
5. Take frequent breaks to rest muscles and to stretch. At least one 5 minute rest every 30 minutes is adequate to perform self-massage and stretching for the muscles.
6. Begin a daily log of your activities and symptoms. In the log, state the symptom and its severity, along with activities that may have brought on the complaint. This enables you to begin understanding why you have this injury. Other items to log are: water intake, medications taken per day, dietary intake, and other factors that may play a part in your condition, such as emotional factors and stress level.
7. Watch your posture while working. If you have questions regarding specific work-related postures, ask the doctor.
8. Cold environments will worsen RSI conditions because of decreased blood flow to the tissues. If you find yourself in a cold environment, take steps to correct it. (i.e. wear gloves, buy a space heater, etc.)
9. Check with your local hardware or stationary store for rubberized materials that allow easier grasping of objects, such as around a commonly used pen, or pliers, etc.
10. Avoid hobbies that flare up the condition, such as sewing, and sports involving heavy shoulder and arm use (like racquetball).
11. Do not perform weight lifting exercises to your arms, hands, or shoulders when the condition is in an inflamed state. This will simply further the irritation and swelling.
12. Use ice, either via an ice pack or by using a Dixie cup ice massage to relieve swollen irritated tissues. When using an ice pack, always place a moist towel between your skin and the ice pack. Ask the doctor on the proper technique for ice massage. Ice should be used for no longer than 15 minutes at a time. Take a half-hour break before applying again.
13. If the muscles in your neck and back are sore and tense, moist heat may be affective in relieving the pain. As with ice, use heat for periods of 20 minutes at a time. Take the heat off for 20 to 30 minutes between sessions. If you feel worse after using heat, apply ice for 15 minutes. This will decrease the swelling present. Avoid heat if this is the case.