Try experementing with a wedge of foam or other padding under your thighs. I have found that my siatic (sic) nerves get pinched from too much pressure on my butt. By supporting my thighs and sitting erect or leaning slightly foreward I distribute the pressure and can extend the time before my legs fall off.

I have seen some very nice seats that people have made that fit their butts perfectly both out of foam and out of fiberglass. Catera kayaks in Tybee Island Georgia (Savanna area) sells an adjustable air padded seat that they swear by.

Pete Strand