If you are on your back
Lying on your back with your knees and low back supported will allow for some bending in the back. If you bring your knees toward your chest, this will increase the amount of bending and may provide more relief. Experiment to find out. You will be passive during sex in this position.
If you are on top
Top partners with back pain may wish to try a modified missionary position in which they kneel. Prop your partner's body up with pillows, which will allow you to draw her or him to you. Or, you could both kneel -- one partner props their weight up on their forearms while the other enters from behind.
Hebert, Lauren, A., P.T. (2001). Sex and Back Pain. Greenville, ME: IMPACC USA.
White, A., III, M.D. (1990). Your Aching Back: A Doctor's Guide to Relief. New York, NY: Simon & Schuster/Fireside.
Kisner, C., & Colby, L.A. (2002). Therapeutic Exercise: Foundations and Techniques.Philadelphia: F.A. Davis Company.