I had tendonitis for awhile, from too much repetitive-motion computer use.  Things that worked for me:

• Attention to posture
• Frequent breaks for hand exercises. Had a reminder program called 'xwrists', I think.
• Split keyboard.
• Use different hands for the mouse. (I eventually put the mouse on my work computer on the right, the mouse on my home computer on the left.)
• A scroll-wheel mouse is handy, I find.  For severe cases, a "touch-mouse" that works like a laptop's touchscreen mouse. I used one for awhile, and once you get used to it, it can be quite nifty.

For pain: hot/cold treatments. Ice pack and heating pad, alternately. (Ice reduces swelling, heat relaxes overtense muscles.)

If problems persist: Wrist braces, available at any med-suply store at about $20 each. They're clunky, but they can really save your wrists.

Don't go for cortisone shots  or surgery till you've tried everything else that can help! Save those for  when the problem gets severe or refuses to respond to lesser treatments.