When you find something you regularly use, on sale for a great price, be sure to stock up. Food and other necessities often follow certain sales cycles, where you find certain things are always on sale at a certain time of year. If you can wait to buy the things you want when they are going to be on sale, you can save a ton of money. By stocking up when things are in season, it will be easier to avoid paying full price later.
Mid-season is a good time to start looking at clothes for the season next year or even to get you through the rest of this year. Sears, JC Penney's and others have had their summer clothes on clearance since mid-July. This pattern repeats every season, but the best deals are usually found in August and January.
As for food, always choose fresh produce that is in season and preferably local. You can buy large quantities and then can or freeze them for use in the "off-season". Other food items to note sales for are: frozen foods in March (Frozen Food Month); ground beef, hot dogs, soda, and condiments in late-June/early July; and baking goods (flour, sugar, chocolate chips, etc) and turkey in November and December.
In January, we tend to find sales on gym equipment, diet plans, diet foods, and athletic wear. Paint and cleaning products tend to be on sale in late-March and early April. Linens can often be found for great prices in May via "white sales". School and office supplies are often drastically marked down in July and August. The best candy sales are before Easter and Halloween, though I generally get it on clearance AFTER any holidays. The new model vehicles come out around October so dealerships reduce the price of the current year's models at that time. Toys and gifts can be found on clearance at really low prices in January.
All in all, the best way to save money is shop when there are sales. Try to plan ahead and you will save even more money.