My '85 Trek 460 has a shock absorbing "System 1" seat that is fairly narrow and not too thick. It's great on the rump, really. For the tailbone pain I usually do what was already suggested in standing up periodically. On downhills I'll stretch back a little and then reposition back into "the groove" when resuming the cadence/pace. I can do 25mi in the saddle without a break by doing this, though when it comes time for that break things are bit uncomfortable back there. And that is the nature of biking in my world.