Ergnomics:

• Ergonomic keyboards are, in fact, wondrous.
• As are mousepads with a gel wrist-rest.
• While you're at it, get another wrist-rest for in front of your keyboard. (You may be tempted by the ones made with tiny beads that they sell in Office Depot. I was, and can tell you that they suck. Gel is much more supportive.) Alternatively, roll up a towel and stick it there.
• Sit in a chair that keeps you from twisting around sideways. (This is a huge problem for me.) A decent desk that is the right height for you (VERY important for us tall wimmins) and a strong chair with good lumbar support make a world of difference. My chair frame is made of steel and is great. I got it at a refurb office furniture place for about $80. Gawd knows what it would have cost at Depot.

• Learn keyboard shortcuts. They’ll save you a lot of pain. I’m still working on this, but a few days ago I made myself do a long series of screen-shots using only shortcuts, and I came out the other end with minimal pain.
• Switch-hit. Learn to mouse with both hands and trade off regularly. You’re using your dominant hand for all sorts of stuff throughout the day anyway, so give it a break.

• The single best thing you can do is do an exercise that really stretches out all the muscles all the way along your arms, backs, and legs. Yoga is great for this — and I often tell people that Sarah Ivanhoe’s Candlelight Yoga is really Writer’s Yoga. It’s gentle enough that anybody can do it, but it really gets things loosened up. A simple long walk with lots of arm-swinging will also help, but it won’t give you the same effect as lots of stretching.
• On days when you know you'll be doing a lot of mousework, start things off with a low dose of ibuprofen. This is important not just for pain relief, but because it acts as an anti-inflammatory. If you minimize muscle inflammation in the first place, you’ll be a lot better off.
• Wear a tendonitis brace when you're working. You want it tight enough to compress the muscles and thus the inflammation, but not tight enough to cut off your circulation.
• Find a massage therapist you like and go regularly. (Do not procrastinate by telling yourself that your body is too fat/too thin/too old and you’ll go when you feel prettier because you don’t want anybody to see you naked right now. Massage therapists see all kinds of bodies, and they do not care. Plus, they probably won’t see you naked anyway, at least not all at once.)
• You can also do self-massage in between appointments. For mouse pain, the stroke you want is working straight across with your thumb across the inside of your lower arm right below the elbow. (That's a very convoluted technical instruction, but I don't know how to say it differently.) Only go one way -- if you go back and forth, you'll just inflame it. I found that out the hard way.
• Applying heat to the elbow and cold to the back of the shoulder also helps me on really bad days. (Not at the same time, though.) Don’t be afraid to ice pretty much any inflamed area.
• If you're accustomed to deep tissue work — and thus pressure points and pain-with-a-payoff — then you can do some similar work at home. What you do is put two tennis balls in a tube sock. Lie on the floor with them under you, and position things so that you have one tennis ball on each side of your spine. The goal is to eventually move them down along your spine to your tailbone. When you hit a place that really hurts, then stop and work through the pain until the muscles unclench. You can also work side-to-side across your lower back and your glutes.
• Speaking of which, don’t ignore your glutes. Everything is connected, and if they’re tight then your back will most likely tighten up too.