You are in the middle of crafting your opus. It is the greatest thing. Ever. And then, the first twinge. Pain? Numbness? A bit of stiffness in the fingers? Uh oh. You're skating on the thin ice of repetitive strain injury. Something needs to change and fast. Here are a few options (all of which I admit to using and possibly need to use more)
1. Switch to an off handed mouse. Seriously. Your main hand is already doing the lion's share of the work and the supporting musculature has a bunch of bad habits surrounding repetitive motion. Guarding. Tightening. This is all exacerbated if you carry your stress in your shoulders. Circumvent that by moving your mouse manipulation to your off hand. It's good for your body and it's good for your brain. It just won't feel like it for the first few weeks. Once you acclimate though, you may never go back. I didn't. Using an off hand mouse has the added benefit of irking anyone who's trying to use your machine.
2. Get up. If you don't have a treadmill desk (I wish, but I don't) then use a standing desk. Mine is a fancy feline play ground model. Meaning that my laptop goes atop the kitty condo. It just barely fits, but it fits and is the right height for me to stand and work. Sometimes, this means I am required to take a writing break to play with whichever cat has taken up residence in the condo. I'm not sure that's ergonomic, but there you go. If you're really, really coordinated, change it into a DANCE desk. Just add MP3 player and ear buds. Dance and write at the same time. There's some recent research suggesting that people who dance live longer.
3. Stretch. My stage combat instructor liked to say, "Long muscles are strong muscles." There might be a limit, but his point was that stretching is your friend. Tight, short muscle fibers are prone to tear injury. Overstretch injuries, while possible, are far less common. The great thing here is that you can legitimately find a great message therapist - someone who does body work, who can show you how a knot in your mid-back translates into burning pain in your wrist. All while working that knot out of your back. You now have a legit excuse for a massage once a month to prevent injury. Score.
4. Strength training. Most of us over train the front of our bodies. It's a common thing. We slouch before our computers. We hunch over dinner. We pour ourselves onto the sofa in front of the TV. There's a permanent hunch in our backs between the shoulder blades that pulls our spines out of alignment and shortens the muscle structures supporting the shoulders, arms, neck, and head. Working the back muscles to build strength counteracts the effect of forward pull. A solid strength training program will put the spine and muscle systems back in line so that no one part of the body is taking the strain.
5. Rest. Muscles are meant to move. But they are also meant to have enough rest to repair the micro-tears that come from activity. Learn relaxation technique. Not so much that you're trying to become Zen or anything. The point is to learn how to drain every last bit of tension out of a muscle fiber, to stop holding. Most of us in this modern world wouldn't know relaxation if it walked up and smacked us in the kisser. Sleep, while necessary, isn't the same thing. It is entirely possible to sleep without having relaxed. It is also possible to relax without sleeping. But combine the two and you'll find sleep improves. So does muscle recovery. And if you DO injure yourself, the ability to relax the tissues surrounding the injury will speed healing.