Try sleeping with two pillows between your legs, positioned width-wise, one between the thighs and the other between your knees and calves. Your top leg is less likely to slip over and onto the surface of the bed, causing a strain at the sacroiliac joint. There are other positions to avoid, one of which is what I call "stork" sleeping: lying on your back, one leg straight and the other hip and knee bent and out to the side. If you get up in the middle of the night and experience pain getting out of bed, try this: get onto your side. Keep your knees and legs apart while you push up into a sitting position at the edge of the bed. Upon returning to bed, do the same thing in reverse: keep knees apart the entire time you are trying to get into the sidelying position. Avoid getting up jacknife style.