I have a hip injury, so I have to have quite a construction of pillows to sleep comfortably. Here's what I've learned:

1. If you sleep on your side with your knees stacked, try a pillow between them.

2. If you sleep on your side with your hips twisted so your top knee can rest on the bed, try a small bolster right in front of belly and hip to make it harder to twist like that. I have a big belly that will pull my lower body around, but if I support it with a bolster, my hips will stay more in line, and my back won't hurt as much in the morning.