Use your height as a guide in choosing your bike. Straddle the middle: You should be able to lift the bike up about two inches before the bar hits your crotch. Sit on the seat and place your heel on the pedal. In this position, your knee should be completely extended so that when your forefoot is in the proper position on the pedal, there will be about a 15 degree bend in the knee. A knowledgeable salesperson can help you with this.

In my city, New York, I've found that the salespeople in specialty bike stores are passionate about and devoted to the sport, and want you to walk away happy so you'll be a lifelong customer. Find a with a salesperson you trust, and work with her to get a good "fit" between you and the machine.

The parts of the bike that should concern you are frame and suspension: frame because it holds everything together and suspension because, the better it is, the less strain on your legs and lumbar region. Front suspension is important because the bike bounces like crazy while descending a trail; the better the suspension, the better the bike can absorb the shock.