The legs tilted up and knees against the braces is the way that I was taught and have read to do. I have also had the problem, but only in certain Kayaks. If you really want a good fix, make a homemade custom seat for yourself out of closed cell foam. If you make a simple frame (or just use a pre-existing seat) and build up a mold for your butt under/with the closed cell foam (open cell will collect water and start to grow things and smell bad).
I have not done this but a guy who has been paddling for 20 some odd years told me that is how he stays on the water for many hours (he is in his 60's). I hope that even though this is vague it helps.
Has anyone done this and have tips/instructions?