Crushed ice works particularly well for people who weigh over 200 pounds. Make sure that if you use crushed ice, that it's really smashed up into a fine pieces-otherwise it's like lying on rocks. Don't cover it with a towel because it just won't penetrate deep enough to work. Instead, use a zip-lock bag. However, the immediate pain of the ice is worth the long term relief you'll get from the reduction of inflammation and swelling.
Ice has to be cold, really cold to be effective.
Imagine the thickness of a t-bone steak and how cold you would need to get one side to feel the effect through the other side. That's about the thickness of the your back. The key to using ice is staying mobile and flexible. Do knee to chest and pelvic rock stretches while lying on the ice. Not only does it keep you from stiffening up but it literally pumps swelling or inflammatory edema out of the low back as you stretch.