I get this too. I've been sitting on a small pillow which helps a little. Also make sure you get up and walk around every so often (or waddle as in my case!).
Oh Sangie, glad I'm not alone!! I'm sitting on two pillows right now and my butt is still sore! Walking around does help so I must make more of an effort to wander. It's any wonder I'm achieving bugger all on the workfront!