Size and Fit
The most important thing to get right on a bike seat is the fit. Everyone's body is slightly different, and to get a seat that's comfortable, you'll need to get a seat that properly supports your body. If you reach behind to your backside, you'll feel a prominent bone on either side. These are known as the ischial tuberosities, or more simply, the sit bones. The sit bones are the two points that make contact with whatever you're sitting on, so you'll want to make sure that the bike seat properly supports these two points. The sit bones should be centered on the wide part of the seat so that they are fully supported.
Biking Style
Different types of bike seats are good for different styles of bike riding, and even the seats that look the most comfortable will only be comfortable when used for the style that they were designed for. Consider your riding style and how you're positioned on the bike when choosing a seat. If you race or spend the majority of your time leaning your body over the front end of the bike, you'll want a seat with a narrow nose to prevent chafing against your legs. On the other hand, if you ride a cruiser and spend your time sitting upright, a flat, wide seat will better support your weight. Test riding a bicycle seat in your usual riding position will help you know if it is a good fit.
Women's Seats
Unlike some women's equipment, women's bicycle seats aren't just a pink-and-purple version of men's seats. Women's bodies are obviously shaped differently than men's, and bike seats are designed to reflect those differences. Women's seats tend to be wider and shorter to provide support for wider hips. While shopping for women's seats is a suitable place for women cyclists to start, it's still important to get the right seat based upon specific fit and style. Since every woman's body is shaped differently, some women might find that a men's seat is more comfortable.
Cutaway Seats
For those that find that a regular solid bike seat irritates the genitals, cutaway seats are designed to decrease the contact area and provide relief. These seats feature a cutout or channel toward the front to limit contact and create a more comfortable ride free of pressure and irritation.
Padding
Bicycle seats use various amounts of padding in their construction. While purchasing a soft bicycle seat with lots of padding is always a temptation to sore cyclists, too much padding can be detrimental. If a bicycle seat has too much padding, it will compress over time and cause pressure points. A bicycle seat's function is to support your weight, so a seat with less padding can often do a better job than one with too much, especially for longer rides in a forward position. Gel seats are available for people who find regular saddles too hard and uncomfortable.
