Next time you reach for a low-fat item at the grocery store, compare it to the "regular" version of the product. In many cases, you may find that there's little difference in the number of calories. You don't need to stay away from all reduced-fat products, but compare the labels! A few examples, courtesy of the National Institutes of Health:

FROZEN VANILLA YOGURT (1/2 cup)
Nonfat = 100; Made with whole milk = 104

PEANUT BUTTER (2 tbsp)
Reduced fat = 187; Regular = 191

CEREAL BARS (1.3 ounces)
Low-fat = 130; Regular = 140

CHOCOLATE CHIP COOKIES (3 cookies/30g)
Reduced fat = 118; Regular = 142