I have the same problem. I'm pretty sure it's due to slight scoleosis that is getting worse as I get older. Anyhow, over the years I've tried a number of things which have helped. Now numbness sets in after about twenty miles rather than five. I went from an old Avocet saddle to more modern ones. I have a Selle Italia SL Flow which I like very much and a Specialized Phenom on the other bike, which is also very comfortable. Having the right width is important for keeping the circulation going, and I find that a flatter saddle is less likely to cause numbness. Shoes: again for the sake of circulation I now wear slightly looser, more padded, less racy shoes (Bontrager Race; I think they're the bottom of the line, but they're comfortable). 79pmooney's suggestion of pointing the seat slightly to one side should also help, as would a thorough fitting, of course.