In reply to Hillseeker:
Saddle height and position would be the first place to start. For saddle height, one decent test is to clip in and ride one footed, then to change to the other foot, and then to use both feet but sustaining the one-footed motion. You should be able to maintain a powerful circular motion (or, more specifically, a motion a bit like a bull pawing the ground - drive, scrape and up). If you can't get any power into the scrape, your saddle is probably a too high - move it in quarter and eighth of an inch increments at most. Your saddle should be flat or slightly nose down - you shouldn't slide off it if you ride with your hands off the bars. It may also be that your bars are too low in comparison to the flexibility of your back
If all that's sorted and the problem persists, try a cut out saddle. Fabric Scoop suits me, and one version has a cut-away groove. they are also very well priced. Personally, I don't have a problem, except when I settle down to a really long, long drag up a hill (an hour or so plus), but less so with a cut-away saddle.