The most important tool in your toolkit for NaNoWriMo are your arms, wrists and fingers. From about now through the start of November, you should be resting them whenever possible. Of course, you may have a day job just as I do, where there is no way to avoid using a mouse and keyboard. But you can stop playing games on your phone computer or gaming system, avoid unnecessary typing on social media, and adopt the best posture possible. This website has the most complete information I had found on avoiding repetitive stress injury by adopting the right posture, and trying to recover from it once you develop it.
During NaNoWriMo itself, remember to take frequent breaks from the computer to relieve wrist pain, and eyestrain. Ice packs are also helpful, hot packs are less so, because they cause the joints to swell even more while the cold packs reduce the swelling. If you sleep on your side, it is best to not sleep on your dominant hand. You can also try dictating text, as I’m doing right now. The first time I tried Microsoft Office speech functions was around 2006 or 2007. It was practically unusable back then. This version almost seems workable. I hope to dictate at least part of my NaNoWriMo project this year. I guess we’ll see how this experiment goes.