Like saverintraining said, it's all about matching the coupons with sales. It's also important to plan meals around whats on sale that week. When you find something cheap, stock up on it so you won't have to worry about buying it later.
If you read some of the success stories, you can see how people match coupons with sales. For example, I went to Publix a few weeks ago and they had Velveeta mac and cheese on sale for $1.49. That is a good price in itself as they usually go for about $2.50 around here. But I had 10 coupons for $1 off 1. So I got 10 boxes of mac and cheese for $.50 a piece. Now I won't have to worry about mac and cheese again until the next sale comes around
It takes time and practice. I'm no expert by any means since I've only been doing this for a couple of months. It takes a lot of patience also. Keep an eye on the sales and always keep a calculator handy so you can compare prices and see what is the best deal.