The Wacom Bamboo Pen Tablet is a low cost ergnomic input device that I use. I hold the stylus like a hold a pen and lightly move it around. I wrap the stylus in a foam insulation so it is about 1.25″ in diameter. This makes gripping it a lot easier. The key to a more ergonomic input device is to try to keep your hand in as close to a natural position as possible.

Another input device I switch off to is:

The Adesso USB EasyCat Touchpad is another low cost ergnomic input device that I use. I use my finger to lightly move the cursor around and programmed its buttons to do tasks that require multiple clicks.

The 3M Ergonomic Mouse is another device I use. Your hand stays in a more natural position and you use the more powerful thumb to click.

Again, these will not solve all RSI problems.  You need to take breaks, do stretching, and minimize computer use whenever possible.