Tendonitis happens because of overuse. Overusing a tendon—for example, typing for hours at your desk, thrashing your guitar for hours, or doing repeated movements in a sport, can create very tiny tears in muscles and tendons. Some tears, which are normal, are collectively called microtrauma. Microtrauma that is unaddressed, or done in excess, can lead to tendonitis.
How can massage help? Massage is a helpful tool for relieving the tension and pain associated with tendonitis. Massage increases bloodflow to your body's tendon attachment areas - so not only will a relaxing session with your therapist be good for stress, but also for the tension and inflammation linked to tendonitis. Also, talk with your therapist either before or after a session to learn how you can maximize the effects of massage on your injury.
How can you avoid tendonitis? Proper rest, hydration, and also stretching can be good weapons in the fight against tendonitis. Be more aware of how you hold your hands when you are doing everyday activities, like typing and exercising. If your job is a little 'technology intensive', make small changes to your workspace. Get a mouse pad with a hand rest for the tendons in your wrist. Make sure your chair is not too high or too low.
As always, if you're concerned about tendonitis, consult with your doctor for more information.