Despite years of my father nagging me to sit up straight, I have horrible posture (sorry Dad!). I also have poor typing habits, which is why all my keyboards and mice have wrist rests nearby. Whether you're using one at a keyboard or at a mouse, wrist rests are designed to keep the wrists in a straighter posture and reduce pressure on the wrist. When choosing a rest, pick one that your hands travel easily over, that is easy to clean and that isn't too soft. Keyboard rests should be positioned so the surface of the rest is level with the front edge of the keyboard; it should also be relatively narrow. You don't want a deep rest that causes you to reach farther to the keyboard.
Also, don't be fooled into thinking that you're stuck using some gel-filled, boring orthopedic-looking device from an office supply store. There's no reason you can't snag a livelier option like a wrist rest in the shape of a delicious donut. Go high-end with sheepskin. Or, there are several animal versions out there as well including oblong kitties and cute snakes. For meme-lovers, there's a NyanCat keyboard wrist rest available on Etsy (as well as several simpler but pretty pillow rests).
However, as with most things in life, it's easy enough to just do it yourself. Grab a (clean, colorful) sock, pour some rice in the toe, add any essential oils you might like and tie it off and you've got yourself a homemade wrist rest. Really, almost any fabric bag you have laying around will do the trick; I myself was lucky enough to convince my liquor store guy to give me several Crown Royal bags a few weeks ago so I just used one of those. If you're savvy with a needle and thread, you can probably whip up an easy keyboard/mouse rest combo in a matter of minutes, it's pretty straightforward in theory: just sewing fabric bags, filling then stitching shut. If you're up for a slightly more involved project, consider this tutorial for the "adorkable" keyboard cat, or this felted fruit creation. No matter which route you choose, your wrists are sure to thank you. Images courtesy the Breadou.com, and Etsy sellers Mr. Robot Hero and Stitch Giftery.