I agree about lying on the floor with the small of your back pressed into the floor - hug your legs to your abdomen.
Do a LOT of abdominal exercises - crunches, etc. but do them carefully and properly. Spread them out over the course of the day. Make sure all of the strengthening and all of the effort involves your abs and not your back. The more you strengthen your abs, the less pressure you will have on your back and sciatic nerve. I've had this for years and it's always better when I exercise gently but frequently.
Try not to do anything that strains your back - carrying heavy stuff up stairs, groceries, etc. I know, I know - it's hard to get around that!
I also take a really good supplement program (comprehensive, not just single ingredients) and the problem is virtually eliminated. It heals you from the inside (vs. applying heat or ice from the outside) and quiets those overactive nerve cells. It's all food, no drugs and I have a lot of contact from nutritional conferences who've gotten sciatic relief the same way.