If the scale is stuck, don’t automatically blame your weak willpower. There are a host of surprising things that can keep you from slimming down. We’ve highlighted six diet problems that can send you into eating overdrive and the easy ways to spot and overcome each one.

1. Snack packs.

There’s complicated psychology at work in those 100-calorie snack packs at the grocery store. They are based on something consumer scientists call unit bias: The bigger the size of the serving of any given food, the more you will eat of it. Reduce the size of the package, then, and it seems like you would eat less. Right? Not if you’re a dieter.

A study from Arizona State University found that non-dieters consumed fewer calories when they ate M&Ms out of a mini-pack, but dieters consumed almost twice as many calories when they ate M&Ms out of snack packs compared to eating out of traditional packaging.

“Consumers perceive the mini-packs to be diet food,” the authors wrote. “For chronic dieters, this perceptual dilemma causes a tendency to overeat, due to their emotion-laden relationship with food.” In other words, the snack-packs seemed “safe,” so dieters felt free to eat more of them.

Another study, from Tilburg University in the Netherlands, that tested the effects of packaging size on the consumption of potato chips in college students came to the same conclusion. The Dutch study found that participants were more likely to regulate their eating when they opened a large bag of potato chips instead of a snack pack. Both studies were published in the Journal of Consumer Research. 

Slimming solution: Instead of stocking your pantry with 100-calorie snack packs, use a plastic baggie to implement your own portion control. Measuring food yourself will force you to be conscious of how many calories you’re eating and reinforce the decision to stop when you hit your pre-planned limit.

2. Diet soda.

Regular soft drinks are the biggest source of calories in the American diet, and numerous studies have tied them to weight gain. Yet, even the calorie-free kind may lead people to put on pounds. Researchers found that each can of diet soda a person drinks daily increased a person’s risk of becoming obese by a whopping 41 percent. The study was published in the journal Obesity.

“We didn’t prove that it causes weight gain, just that it’s connected to it,” notes study author Sharon Fowler, faculty associate in the division of clinical epidemiology at the University of Texas Health Sciences Center in San Antonio, though she admits she doesn’t know why diet soda might cause weight gain. “One possibility, which we haven’t studied yet, is that there’s a chemical in diet soda that increases appetite,” she says.

Many dietitians point to another, more obvious reason why some diet drinkers gain weight: They use soda as justification for bad food choices. “I’ve had clients tell me that they figured it was OK to have a cheeseburger and fries because their soft drink didn’t contain calories,” says New York City-based dietitian Tanya Zuckerbrot.

Slimming solution: Whenever possible, swap your diet soda for water, unsweetened iced tea or seltzer with a splash of juice. If you can’t live without it, try to cut back to no more than one every other day – or ideally, save it as a treat, advises Zuckerbrot.

3. Too many choices.

You’d think that having lots of different types of food on your plate would make you feel more satisfied, but in fact, the opposite is true. A study at the University of Pennsylvania in Philadelphia found that people given a bowl of 300 M&Ms with 10 different colors ate 43 percent more than those given the same amount of just seven colors. “When you have fewer items, it’s easier to grasp how much you’re consuming,” says Judith Beck, PhD, clinical professor of psychology in psychiatry at the University of Pennsylvania, and author of the Complete BeckDiet for Life (Oxmoor House, 2009).

Slimming solution: Limit yourself to three or four choices when you’re eating at a salad bar or a buffet instead of taking a taste of everything, advises Dr. Beck. “It also helps to decide what types of food you’ll choose before piling your plate; for example, lean protein, a green salad and some whole grains.”

4. What’s in a name?

In a word, calories. There’s a reason restaurants give their desserts tempting names such as Molten Chocolate Lava Cake. Those luscious sounding words work.

A study from the University of Illinois found that when a piece of plain chocolate cake was given the name Black Forest Double Chocolate Cake, people said it tasted better and were more likely to buy and eat it than when it was called Chocolate Cake. Conversely, the researchers found that when foods are labeled as healthy (think “contains soy” or “low-sodium”), people assumed they wouldn’t taste as good.

Slimming solution: Be aware of this phenomenon so you’ll be less susceptible to it. “Remember that you can’t depend on the name of the product to tell you how satisfying it will be,” says dietitian Dawn Jackson Blatner of Chicago.

Remembering a time when a tempting description suckered you into ordering a dish that sounded better than it actually tasted may also provide a reality check in a moment of weakness.

5. A health halo on fast food restaurants.

Eating at a restaurant that bills itself as better for you may backfire if you aren’t careful about what you order. According to a recent study from Cornell University in Ithaca, N.Y., people who go to a fast-food restaurant with a healthy image (think Subway) often end up eating more calories than those who go to traditional fast-food establishments like McDonalds. “When people think a type of food is good for them, they think that it can do no wrong, so they don’t pay attention to their portion sizes and don’t check to see how many calories they’re consuming,” explains Blatner, author of The Flexitarian Diet (McGraw Hill, 2008).

Slimming solution: Do your homework. “Before you go to a restaurant, visit its website to find the nutrition information for its items,” recommends Blatner. “That way, you can make an informed decision when you arrive. ”Healthy choices should have approximately 500 calories and less than 6 grams of saturated fat.

6. A “free” day.

Giving yourself a “free” day on the weekend to eat whatever you want isn’t an effective strategy, although it’s often touted as a way to keep your diet from being too restrictive. “People tend to overeat and make very poor food choices on the ‘cheat day’ and they feel deprived the rest of the week as a result,” says Beck.

In fact, researchers at the National Weight Control Registry found that people who shed pounds were more likely to regain them after a year if they allowed themselves to eat more on weekends and holidays.

Slimming solution: A little wiggle room in each day’s calorie count can keep you from feeing deprived. “Give yourself 150 bonus calories a day,” advises Beck. “That’s enough to satisfy cravings, but not so much that you’ll blow your healthy eating habits.”