Depends on what you want to do with it
If you're looking to resell a domain, go for short urls (4 letters or less), premium letters, generics, brandables & pronounceables.
If you're looking for something unique that you want to start, go for something catchy, brandable and very web 2.0 (5 letter domains are "in" right now for that sort of thing).
If you're looking to tap into a saturated market, go for generic dot coms.
Hope that helps a bit.