100 sit ups a day and u will want a wasty [}:)]

I use a dakine wave seat, it's really a waist with seat attachment, spreads the load out a bit more, with plenty of lumber suport. I find a good blast with that helps to sort out any back pain I may have.

hey WindyMiller, I am not sure seat harness are better for back support, I would tend to say the reverse (=waist harness are )
Personnaly, I had bakc problems and a low back surgery, and find that a waist harness is definitely better for back support. Even the doctors, surgeon, physios sometime give low back support waist belt that looks like a waist harness to me.
Also remember that you dammage your back more by being in a sitting position that standing (so being completely crunched, overpowered, bouncing on the chops can't be too good), being able to hook and unhook easily must be better for the body in general.

but if you are really keen on nappies, maybe something like what decrepit mention could be good.

I sailed a seat in the waves until very recently. The seat is definately more comfortable. Especially on long hauls. The advantage of the waist is you can unhook really quickly. if i had back drama i would use a seat.

Haven't tried many seats but the ones I have tried, played merry hell with my back. Waist or chest is the opposite sorts any of my back pains out.