There really is no special secret or anything. At least there isn't on my Jeep Wrangler. Basically keep it clean with soap and water. I think there is some vynal cleaner that can be used on it, but you have to be careful what chemicals you apply to a convertable top because many will clog up the pores in the material and end up being harmful. I would say just keep it as clean as possible, especially when putting it up and down. Having dirt on it when you fold it up or down will cause extra wear.
How long they last really depends on your climate, and how much time it spends in the sun, how often its opened/closed. I run a hardtop in the winter on my Jeep which greatly increases the softtops lifespan. My uncle has been driving convertable cars a long time, and he had a LeBaron that didn't need a new top until it was a bout 10 years old. So if it is representative, they do last a while. PS, he drove it year round too, exposing it to heavy snow, salt, wind, etc etc.