After watching Extreme Couponers I attempted to coupon once. I think I saved a grand total of 3.20. I deemed the whole experience not worth it.
It's very worth it if you have the initial money/time to get started. Basically the theory is that you buy it/get it when it's on sale and stockpile (not always meaning 1,000 tubes of toothpaste, but whatever your family needs to get through a month or two).
In a super simplified situation:
Every week, I buy milk, shampoo, can of soup, box of cereal and a head of lettuce. At $2.00 each, my bill comes to $10/week/$40 a month Every week, I have a coupon for $.50/off for the shampoo, soup and cereal ($1.50 off total bill).
Well, this week, cereal is on sale for $1.00, so my coupon makes it $.50. I buy 5 boxes (to last through the month) and my total cereal expense for that month is $2.50. The next week, I go to the store and shampoo is on sale for $1.00 a bottle and is now $.50 (with coupons). I buy 5 and spend $2.50 that month on shampoo. The next week, soup (same situation). As each week goes by, instead of spending that $2 on those items, I just take it out of my closet and then I replenish the next sale.
I could use my coupons every week, but I'd still be spending $7.50 (vs my $10). It's a savings, but not as much as it could be. At the end of the month your grocery budget breaks down to:
Without coupons $40
Using one coupon per item, per week, $30
Stockpiling when you combine a coupon with a sale price $22 (if my math is right, I'm bad at math sometimes).
Then you get into coupons at walmart/places that give you overages and double coupons, etc and that's more complicated, but still worthwhile. The biggest "trick" to couponing (besides stockpiling when it's on sale) is to learn to cook what you have and not find coupons to what you want to buy. If chicken is on sale this week, we eat chicken. If this brand of shampoo is on sale, we use it. While most of the coupons tend to be for health/beauty/cleaning or snacks/"junk", there are coupons for "real" food too.