Laura Ries carries on in her father's footsteps, writing terrific stuff about positioning. Here's her checklist to stimulate your thinking when you're getting ready to work on where to position a product (as I am, just now).
I'm a big fan of market-segmentation and positioning, especially in the early stages of a product's life. I don't think you can come up with too many ways to segment a market when you do this kind of analysis. Laura describes several new dimensions that I'll add to my standard list.