There are lots of good suggestions here. I really like the notion of getting out of the saddle regularly to improve the circulation. I used to live in the flattest land in the West (Sacramento Valley), and everyone seemed to figure this out pretty quickly since there were no hills that would encourage standing anyone who didn't get out of the saddle every so often would find their rear and often their knees would be unhappy.

Have you considered that you are just not riding hard enough to get the load off your saddle and onto your pedals? The faster you ride the less of a beating your rear end will take.