Pillows, pillows, pillows, LOL. Arrange them however you are most comfy. If you like sleeping on your back you can prop yourself up into a semi reclined position with pillows - just a little bit is enough to take the pressure off aparently.
You can buy moulded pregnancy pillows too - full body ones that you can arrange a few different ways to keep yourself comfy. (I didn't find out about them til late in my pregnancy... I'm going to get one for my next one for sure!)