When you are getting ready to lie down, don't rush it. Keep your back straight, and think about lowering your body without twisting your back.
Supporting your back
Keep your back straight, and use your arms to help you lower your upper body to the bed. Your arms can support your upper body weight and keep you from twisting your back. The idea is to keep your back in one straight position as you lower your upper body and raise your legs.
Keep your back straight after you are lying down. You may want to put a pillow between your knees to keep your back in a neutral position. If you roll onto your back, you may want to put a pillow under your knees.