I'll chip in and go with saddle choice and bike fit like a lot of other people already did. I found that once I got my Brooks Imperial and broke it in, I can last just about forever in that saddle. On my old saddle or other bikes without that Brooks, I get saddle pain a lot earlier.
I also try to vary my riding on longer rides. I find that standing up to pedal now and then really helps relieve the pressure and refreshes me to sit back down again for awhile. You can also change gears to take up an easier cadence. I personally think that the harder gearing you are pushing, the harder it can be on the butt. Spinning, for me at least, is another method I use when I my butt is getting unhappy with a ride.