An inflatable seat might be the answer, but it will also raise your centre of gravity, and make you less stable. If you're comfortable with that, go for it.
When I've had this problem in the past, I've found that it was due to two things:
1. Keeping the knees in the 'locked' position all the time - ie being fully braced under the deck. This inhibits a fluid stroke and pushes your butt hard into the seat. And it isn't necessary. You can always find the braced position when you need it.
2. Having too passive a stroke. Bringing the legs more into the stroke will naturally keep things moving in the seat, preventing numbness.
Hope that helps.