You're overstuffed from dinner, yet you can't resist ordering the Double Dark Chocolate Two-Layer Cake for dessert. You devour an entire bag of barbecue-flavored potato chips at one sitting when you felt like having only a few. Everywhere you go, from the "big box" retailers to your own desk at work and kitchen at home, environmental cues encourage you to eat more than you need -- or even want.

Researchers are discovering just how powerful an influence these cues have on your tendency to overeat. And you don't have to overindulge by much to gain weight. "For most of us, the imbalance between our energy intake and expenditure is just 50 calories daily," says Brian Wansink, Ph.D., director of the Food and Brand Lab and a professor of nutritional sciences and marketing at the University of Illinois at Urbana-Champaign.

"Ninety percent of people who are gaining 1 or 2 pounds per year could maintain their current weight if they ate just 50 fewer calories each day," he adds. If they ate just 100 fewer a day, they'd lose weight."

The single most powerful cue to consume extra calories is the simple fact that they're there. "People find it almost impossible to resist the ready availability of food," says Pennsylvania State University food-selection researcher Barbara Rolls, Ph.D., co-author of The Volumetrics Weight-Control Plan (HarperTorch, 2003).

She cites a study in which people were served soup from a trick bowl that never became empty; it refilled itself from a reservoir hidden under the table. Everyone who ate from the bowl consumed more than their usual portion of soup. When told about the trick, some went back to their normal portions. But others just kept eating, unable to say no to food that was right in front of them.

Other potent eating cues -- whether we're hungry or not -- include any sounds, smells, activities or times of day we associate with eating, such as hearing the lunch-truck horn at work, as well as food ads and low food prices. And once we're prompted to partake, it's difficult to stop. "We do a good job of being conscious of what we eat, but we spend a lot less time thinking about volume," says Wansink. "It is possible to fat-proof your environment, though. The key is to realize you're influenced by your surroundings and choose accordingly."

Here are six of the most common pitfalls you're likely to encounter, along with ways to avoid them.

Pitfall 1: Economy-sized anything

Large container sizes can prompt you to prepare or eat more food than you want. When Wansink gave women a 2-pound box of spaghetti and told them to remove enough to make dinner for two, they took out an average of 302 strands. Given a 1-pound box, they removed only 234 strands, on average.

Eat directly from a big package or container, and you'll probably consume about 25 percent more than you would from a smaller package. Unless it's a snack food like candy, chips or popcorn: Then you'll likely eat 50 percent more! In one study, Wansink gave people either a 1- or a 2-pound bag of M&M's and either a medium- or a jumbo-sized tub of popcorn. On average, they ate 112 M&M's from the 1-pound bags and 156 from the 2-pound bags -- and they ate half their popcorn, whether their tubs were medium or jumbo. "When a container is large, people have trouble monitoring how much they're eating," Wansink says.

Solution Buy smaller packages. If you prefer to buy the larger economy size of a product, repackage the food into portion-sized containers based on the label's serving size, especially if it's a snack food. That way you'll know how much you're eating.

Pitfall 2: Convenience & availability

Keep snacks in sight and at hand, and you'll reach for them all day long. When Wansink placed chocolate candies in plain view on office workers' desks, they ate an average of nine pieces each per day and tended to lose track of how many they'd eaten. When the candy was in their desk drawer, they ate only six pieces; when it was out of sight six feet from the desk, they averaged just four.

Rolls tells of a similar type of experiment in a hospital cafeteria: When a lid was kept on an ice-cream cooler, only 3 percent of obese participants and 5 percent of normal-weight ones chose ice cream. When the lid was removed to allow people to see the ice cream and reach it more easily, 17 percent of obese people in the study and 16 percent of lean ones selected it. "Whether or not we need food, when it is put in front of us, we eat it," Rolls says. "And many of us eat it all."

Solution Hide tempting treats. Don't put unhealthful snacks where you can see them. If you must have something within arm's reach, make it celery or carrot sticks, or fill a fruit bowl and keep it close at hand.

Pitfall 3: Optical illusions

People perceive tall, slender glasses as holding more liquid than short, wide ones, even when both hold the same amount. Wansink had people pour fruit juice into both types of glasses and found that they drank nearly 20 percent more from stubbier glasses, though they perceived themselves as drinking less. "Our eyes tend to overfocus on height, causing us not to see how much volume a short glass contains," he explains.

Solution Think tall and skinny. When enjoying high-calorie drinks like fruit juice, smoothies or alcoholic beverages, use tall, narrow glasses. You'll think you drank more than you actually did.

Pitfall 4: Out-of-control portions

Most people eat more when they're served more. In one of Rolls' studies, restaurant diners were given different-sized portions of baked ziti. When served an additional 52 percent, they ate 45 percent more. And when Wansink gave people stale-tasting 10-day-old popcorn, they still ate 44 percent more from large buckets than medium-sized ones. "Portion cues can even overcome taste," he says.

Solution fill up on smart choices. Nobody ever got fat from eating extra-large portions of salad greens. "As long as you choose the right foods in the first place, you needn't eat less," Rolls says. Big helpings of foods containing a lot of water, such as vegetables, fruits and broth-based soups, can provide satisfying portions with few calories.

Pitfall 5: Bargain-basement food prices

Most fast-food restaurants offer such great deals on supersized portions that you feel foolish ordering smaller servings that cost more per calorie. "When two pieces of something cost less than one, it's clear that the pricing system is wrong," says Simone French, Ph.D., an expert in obesity and eating disorders at the University of Minnesota in Minneapolis. One of her studies found that lowering the price on vending machine snacks by as little as a nickel spurred more sales than labeling snacks lowfat. "You need to be vigilant," French says. "Everywhere you go, you'll find food sellers undermining your desire to make good choices."

Solution Check your bottom line. Ask yourself if getting your money's worth in the form of huge portions is more important than reaching your weight goals and staying healthy.

Pitfall 6: Too many choices

Eating a variety of foods is good because it increases the chances that you'll get all the nutrients you need. But variety also prompts overeating (we tend to get bored with familiar tastes and stop eating sooner). In one experiment, Rolls served sandwiches with four different fillings; people ate one-third more than they did when she gave them sandwiches with their single favorite filling. In another, people who were presented with three shapes of pasta ate 15 percent more than when they were given only their favorite shape. And Wansink found that when he offered people M&Ms in 10 colors, they ate 25-30 percent more than when there were seven colors.

Many people, Rolls says, satisfy their natural desire for different flavors and textures by choosing myriad products -- but ones that are all energy-dense (i.e., high-calorie), such as chips, crackers, pretzels, ice cream and candy. This is a virtual prescription for weight gain.

Solution Indulge your need for variety with healthy foods. Make variety your ally. "Surround yourself with a wide selection of foods that are low in calories but high in flavor, such as fruits and vegetables, beans, some soups, oatmeal and lowfat yogurt," Rolls advises. For example, fill your plate with salad greens and lots of vegetables first, then take small portions of energy-dense foods like meats and cheesy casseroles. Monotony also can be an ally: If you're offered an assortment of cookies, select just one kind and you'll probably end up taking in fewer calories.