Optical: Old-fashioned trackball mice are cheap, but the low maintenance of optical is worth the extra 20 bucks.
Wireless: Manufacturers advertise wireless mice as the next big thing, but they're still relatively expensive. Think carefully about whether you'll actually need to get rid of a cord or use a mouse away from your desk.
Look and feel: Gray mice are passe, so you have lots of options to match your decor. If you're in a store, ask to pick up a mouse and test it to see if it feels comfortable in your hand.
Scroll wheels: Many mice have scroll wheels for moving through windows without having to drag a scroll bar up and down. Some mice have buttons to switch between applications or scroll left to right. These can be timesavers, but they're expensive, and you may find that you don't use every bell and whistle.