i vouch for what marcellus said. you'll be wanting to open up your pelvis and sit with a more forward possie. i did this as often as i could (so for example i sat on the fit ball with my legs to side of it and slightly back...kinda like when you ride a horse) at the dinner table. i also had a swedish ergonomic chair in the house so used that when i was at the computer).

for sleeping, we were told to lie on (preferably) left side and have the right leg swung over as high as it would go...i could almost sleep on my belly in that position. and towards the end of the pregnancy i would use a pillow to support the aching in the hips...

check out spinning babies too (as others have mentioned). it gives a great insight into 'baby mapping' which i followed and was able to use to see where this bubs was positioned.

HTH!