Big suggestion... Wacom Tablet. Here's a sufficient one for most any use:
http://www.wacom.com/productinfo/6x8.cfm
Kind of expensive, but very worth it, both for fluid Photoshop work and just for switching off between mouse and pen occasionally, to prevent repetitive motion problems. If you want it cheaper, look for it (or the equivalent sized version from their Intuos 2 line, which is the one that I have) on eBay.
