Think Quantity
Buy in bulk. You can get things like natural peanut and almond butter, cereals (all bran), rice, whole grain pasta, all kinds of stuff. I even buy my spices in bulk -- way cheaper, too!
--gladiolagrl

Pick Unprocessed Foods
The less processed a food is, the less expensive it generally will be. A head of lettuce is cheaper than a bag of lettuce. Fresh produce is (usually) cheaper -- and definitely healthier -- than canned, as long as it is in season.
-- funniegrrl

Find Farm-Fresh Produce
Does your town have a farmers market? Some places have them year-round, and you can get in-season veggies and fruits much cheaper than at the store, since you're usually buying directly from a farmer. Sometimes you can buy a lot of a particular item if it's close to the end of prime and freeze it, saving even more than if you had to buy it later on.
-- coelura

Avoid Prepackaging
Avoid processed foods like prepackaged macaroni and cheese. It's expensive, high in fat and low in nutrients. It might seem cheap because it's under a dollar a box, but you get very little for your money. Buy your own pasta and add a little tomato sauce.
-- jeanwl

Try "Damaged" Goods
One trick you might try is to go to your local grocery stores and ask the managers if they have any damaged goods they'd be willing to sell cheaply. A lot of superthrifty penny-pincher types do this. You might be able to pick up some things that are still perfectly safe but that the store doesn't want to put out on the shelves because they're not perfect. And if there's a bakery in your area, ask for day-old bread.
-- funniegrrl

Pack Your Lunch
Brown-bag it. I've learned that the so-called "healthier" selections at fast food joints are extremely pricey.
-- gladiolagrl