You also want to think about saddle material and padding. I swear by my Brooks B17 leather saddle. In fact, after I had my first Brooks stolen last year, I swallowed hard, fought back tears and bought a new one to replace it. Leather stretches to conform to the shape of your pelvis and allows your body to slide naturally as you pedal. An added bonus is that you can adjust a tightening bolt under the saddle as the leather softens and stretches over time.
If the leather option is too expensive or not to your liking, by far the most common bike seat is a variation of a hard plastic shell padded with foam or gel and covered with vinyl. Beware that too much padding can result in chafing on your thighs. If you really need extra cushioning, consider padded bike shorts as an option.
So you found your dream saddle. You now know more about bike seat design than you do about gear ratios or bearing grease. You even tried a couple of seats out after test-driving a few bikes at your local shop. There is more. To fully reap the benefits of your carefully chosen saddle, it needs to fit on your bike properly. You can adjust your seat three ways, the most obvious being raising or lowering your seat post. Underneath the seat you will see two rails that are attached to the seat post with a clamp. The seat can be moved forwards or backwards along this clamp. You can also tilt the nose of the saddle.
Finally, some saddles come with springs, and you can even buy seat posts with shock absorbers. This is a matter of personal taste. My thinking on this is that if you have maximized the saddle surface that your body is in contact with and the saddle is properly installed, you should be ready to ride.