Probably the best advice is to try out as many bikes as possible at your LBS. And by try out--I mean don't just piddle around the parking lot---go for a 5 or 10 min ride if they let you. If you just ride in circles in the lot, you wont get a feel for the bike and how it will handle when you get up to speed. Also, be sure to try out the breaks and deraileurs and see how it corners.

Also, just like buying a car, be sure to try out a high end model so you can tell the difference between what you get with a low end model. You may not notice any difference and then there is no point to spending extra $ when you dont need to.