Lots of helpful info in this thread. Here's my contribution.
In addition to using coupons and rebates, there are a few "rules" by which I shop. I've described them below and I've linked to sites where you can get excellent information about the concepts.
If you don't use it, don't buy it. Exceptions:
1. The item is free or a money-maker and will push your purchase over a reward threshold. E.g., Rite-Aid offered a $20 savings reward for purchases totaling $100 in a particular period. I used free and money-maker items to increase my accumulated purchases enough to qualify for the $20.
2. You can give the items to those who are in need. I collect personal care items (shampoo, razors, skin care, toothpaste, toothbrushes, contact lens care, OTC meds, etc. to donate to groups that serve members of the armed forces or their families).
Don't shop for things you need nowYou shop for immediate use items in your larder / pantry / storeroom and go shopping for items to stock your larder. This necessitates having a store of items, but don't despair, that will build up over time.
Shop sales and loss-leaders Yes, even for staples such as bread, eggs, and milk. This is possible because I stock up on sale items (it helps to live in metro areas such as Los Angeles, where there is a great deal of grocery competition). Most times, if you looked in my shopping trolley, you'd think I had an eating disorder because there might be 10 boxes of mac & cheese, a dozen tins of diced tomatoes, 8 boxes of microwave popcorn (a weakness), and 5 boxes of plastic bags. Or 2 18-count cartons of eggs, 5 bottles of salad dressing, and 6 toothbrushes. And each week looks just as odd.
A word about name brands--You don't have to buy name brands, though you might find that they can be more economical when using couponing techniques. I use Tide laundry detergent. It is significantly more expensive than generic or 2nd-tier products. So why do I buy Tide? Because 1. It consistently tops the Consumer Reports ratings for effectiveness (so I get clean clothes without having to use more than the recommended amount or having to rewash my laundry) and 2. there are frequent sales, promotions, and high-value coupons for name-brand products and by using them (mfg coupon + store coupon + rebate) along with $-off store rewards such as Register Rewards or ExtraCare Bucks, and my price book (see below), I can purchase Tide at prices that are lower than the sale price of generic detergent. Even if the price was equal, remember that price and cost are different.
Know the coupon policy where you shop(This may sometimes appear to require an advanced degree

) E.g., Safeway will only double the first of identical coupons. In order to get the maximum discount at stores with such a policy, you must divide your orders so that you don't have multiples of the same coupon in any order.
Don't be distracted at the register You must pay attention in order to catch shenanigans such as: clerks not entering all your coupons, I've caught clerks in Target (there was a recent scandal regarding them cheating customers out of the value of some coupons with dodgy register programming) slipping some of my coupons beneath the register).
Registers not properly accepting your coupons (there is an error beep that will alert you. Sometimes clerks must manually adjust a coupon downward if it is for more than the price of the item; you'll get the item for free, but can't have a "profit". I caught a CVS clerk clearing the error but not giving me the coupon value and trying to pocket my coupon).
Sometimes honest mistakes occur or the clerks have not been properly trained on the official coupon policy.
Organize coupons for ease of location and accessThere are many ways to organize coupons -- and no "right" way -- just customize the way that works best for you (you might need to try a few methods before you find the best fit). Southern Savers have the best descriptions of the various methods.
Know when a sale is really a saleEach week, there is a colourful ad for the market, but not every item is on sale, and not every sale price is a good deal. In a given store, paper towels, salad dressing, ground beef, and pasta may be on sale 2-3 times a month and many more times during the year. They want to sell high and you want to buy low, but how do you know when to buy?
Sales Cycles and Stocking UpTo get the most saving on your purchases, you must know what the best prices are, the interval at which they appear, and where they are offered. This necessitates that you keep track of what you pay for your items. After a period of paying attention to this information, some of it will simply stick in your head (typically for the most frequently purchased items), but unless you posses an eidetic memory, you will need to use a price book. Don't panic, this isn't complex or particularly time consuming. The information you require is printed on your receipt. But since you don't want to have to sort through weeks or months of receipts every time you sit down to your make up your shopping list, you need to put it into an easy-to-use format.
Your price book can be a little spiral notebook, forms you downloaded, an online service, or a spreadsheet that you maintain on your computer. Electronic price books have the advantage of being searchable and sort-able and if you have a smartphone or PDA, you have the advantage of being able to see and enter information wherever you happen to be.
How to Develop a Price Book Online Price Book w/ integrated grocery list and analysisfree downloadable price book spreadsheet (they have a more sophisticated version for sale and lots of other free spreadsheets.
Getting-Started TutorialsSouthern SaversOnline Video Coupon ClassLaura Williams MusingsCouponing-101another Couponing-101Deal / Match-up Sites couponmom.comdealseekingmom.comSouthern SaversCoupon Databases DSMMSQ