Here is the key to making coupons work for you: wait for a sale to use them. Very simple!
Stores use something called "Loss Leaders" you can generally find these products on the front page of each store's ad. The store is offering the item at a rock bottom price. (Beware, not all items on the front page are "loss leaders") They offer these items at super low prices to get you in the door so you will, hopefully, spend lots of money in their store.
By matching a "loss leader" or any great deal with a coupon you maximize your savings. Don't get pulled in by the store's advertising techniques (unless you want to). Just go in get your deals, use your coupons and get out.
Shopping Sales is a great way to save money, but adding a coupon on top of an already low price will save you lots of money.