What are good, safe positions to sleep in while pregnant?

Back and side. Back and side are the preferred positions. On side, place pillow between knees to reduce tension on them. On back, might need 2 pillows to prop up head/upper back, but be careful not to put more stress on abdomen and get uncomfortable. Some will place pillow under feet while on back. Good luck.
On your side. But don't panic if you find yourself in another position when you wake up. Don't go to extremes either about your sleeping position. Tilting to your side gets the most blood to the placenta and is something that makes a difference if you have very high blood pressure, diabetes, or poor circulation in pregnancy.