Make sure you put something under your knees (a sturdy pillow, a rolled up blanket) when on your back. Having your legs flat puts a lot of strain on your back and is uncomfortable. Also propping your arms up on pillows on either side. Basically make a nest so you kind of stay there. Elevating your upper body so you are slightly sitting up can help too.

Have had to sleep on my back and that is not my natural sleep position.