I found that some shims at the front of the seat would help. Not the kind of thing you feel right away, but notice after a long trip.

http://www.xterragalactica.net/mods/raiseseat.html

There is also a product called the Lewis Marketing Lumbar Support. A rubber bladder that you pump up with a bulb. I have not looked, so I don't know what/if it could be adapted to X seats.

dave and xtoy - shims were enuf

PS - we nuked the duplicate post