But I switched to satin pillowcases recently so no more lines...

If you drink lots of fluids through out the day and then more when you wake up they should go away much faster.  Your skin will snap back way faster if it's well hydrated!

Even if you sleep on your back or use a satin pillow case, its possible those creases mean your skin isn't holding enough moisture. I agree with the advice to drink more water. Also try taking a Neocell hyaluronic acid capsule to hold moisture in your skin. 

Sleep on fabric that is specifically formulated for sleep creases. No, not the as seen on tv hype silk pillowcases. Pillowcases such as Dreamskin made Juvetex fabric that are tested and proven to prevent moisture from escaping the skin during sleep. The fabric generates new collagen. No more sleep wrinkles or sleep lines. Very soft too.

 To prevent rest facial lines, the best method is to rest on your again. People who rest in this position don't develop bed sheets facial lines.

If you can only get to sleep, sleeping on your side, then use a cotton or cotton pillow case that allows your face to move across the bed sheets thereby reducing skin wrinkles.

Or, try putting a bed sheets under you legs to reduce stress on your again. But if that's really unpleasant for you, investing in an anti-wrinkle bed sheets is the answer. .