You know how weight loss works: Eat less and move more. But if it were truly that simple, two-thirds of American adults wouldn't be overweight or obese right now, with corresponding health woes ranging from diabetes to heart disease to some forms of cancer.

"The reasons we carry excess weight depend on our culture, our habits, our genes, and our psyches," says John Bagnulo, nutritionist at Kripalu Center for Yoga and Health in Stockbridge, Massachusetts. "Only by examining both the physical and psychological reasons can you find what works for you."

We spoke with several weight-loss experts to identify the most common reasons people struggle with their weight. Identify your biggest obstacle (or obstacles) on the following slides, and use the suggested strategies to start your summer off right.

Considering that our bodies are designed to move, most of us spend an awful lot of time sitting -- in the car, behind a desk, on the couch. "If you sit all day and eat too much, you create a calorie surplus, which leads to weight gain," says Jillian Michaels, resident trainer on NBC's "The Biggest Loser" and author of "Master Your Metabolism."

Working exercise into your routine not only burns calories but also improves mood, energy level, and sleep, all of which can help you maintain a healthy weight.

Each Sunday night, strategize ways to build exercise into your week: Research shows that setting aside time to brainstorm specific ways to get moving can help you stay on track. If you're just beginning a routine, plan to exercise for 30 minutes, four days a week -- a few walks on your lunch hour, a fitness DVD one morning, and a hike Sunday afternoon, for example.

"Gradually build up to four 60-minute workouts per week," says Michaels. She recommends a combo of cardio (such as swimming, biking, walking, stair climbing) and strength training that uses your own body weight (such as lunges, squats, push-ups) so you don't need to invest in equipment or a gym membership.

Try listening to music. When exercisers listen to up-tempo songs, their endurance can improve by up to 15 percent. What's more, tunes can also boost mood and positive feelings about exercise.

Exercise with others. "Lasting lifestyle changes usually require support," says Sasha Loring, a psychotherapist and at Duke Integrative Medicine in Durham, North Carolina. To boost your chances of success, enroll in group fitness classes, or exercise with someone who has similar goals.

Don't mistake your afternoon cookie habit as a mere lapse in willpower. "Physiological addiction to comfort foods is very real," says Bagnulo. The key factor involves serotonin, the neurotransmitter that plays a big role in mood.

"Foods with large amounts of refined flour and sugar trigger artificially elevated levels of serotonin," he explains. "We become dependent on these foods to feel okay."

"If you start the day with highly refined foods, like a bagel or a sugary cereal, you'll crave those types of foods all day," says Bagnulo. "I call it the tsunami effect."

At breakfast, avoid sweetened foods, juices, and products made from flour (such as cereal and bread); instead, power up on protein-rich picks such as eggs, some dairy (if tolerated), nuts and seeds, and whole grains such as steel-cut oats. This helps moderate your appetite for the hours ahead.

The naturally occurring sugars found in fruit don't cause the same spikes in blood sugar and insulin that refined sugar does. The next time you crave something sweet, eat an apple, plum, or some berries.

Note that it may take 20 to 25 minutes to feel satisfied. "You have to give the sugar from the fruit a bit longer to reach your bloodstream," Bagnulo explains.

As you eat, pause every few bites to take a deep breath and ask yourself how the food makes you feel.

"We've become so mental about food -- how many fat grams does it have? -- that we're disconnected from what we're eating," Bagnulo says. "Checking in with how a piece of fruit makes you feel versus a cookie gives you answers about which foods improve your well-being and which detract from it."

If you binge on chips to cope with afternoon work stress or habitually fix yourself a bowl of ice cream after dinner, you create a mind-body disconnect that encourages weight gain. "Anytime you eat when you're not hungry or don't stop eating when you're full, the food you're putting in your mouth has no connection to your body," says Geneen Roth, author of "When Food Is Love."

Eating mindlessly -- while driving, watching television, or checking e-mail -- can cause a similar disconnect, whether or not it's fueled by underlying emotions. That, in turn, often leads to poor food choices and overeating.

"Your body sends clear hunger signals," Roth says, but if you're listening to your emotions instead of your physical cues, you won't get the message and you will overeat.

Before you eat anything, sit down, take five deep breaths, and focus your attention on how your body feels. If you detect true hunger, eat. If not, get up and do something else: Write in your journal, talk to a friend, go on a walk -- whatever helps you process your emotions.

According to a 2008 study, eating quickly and until you're full triples your chances of being overweight. Sharing meals with friends or family is one of the best ways to slow down. You'll chew more slowly, feel fuller, and be less likely to stuff yourself.

To get more mindful about food, eat meals in an environment that promotes calm. Move away from the computer; turn off the TV. "Eating in a relaxed environment allows you to hear your body's cues more clearly," Bagnulo says, which makes you more likely to stop before you reach an uncomfortable fullness.

Metabolism slows down about 5 percent every decade, starting in our mid-30s. But don't resign yourself to the extra pounds.

"For most of us, lifestyle and environmental factors are more important than age or genetics for controlling weight," says Lawrence Cheskin, director of the Johns Hopkins Weight Management Center.

Now's the time to trim empty calories from your diet: A 2009 study at Brigham Young University found that middle-aged women who weren't conscious of eating less as they aged were more likely to gain weight over a three-year period.

"I counsel my patients to think of it not as curbing intake, but as choosing worthy foods with intention," says Beth Reardon, integrative nutritionist at Duke Integrative Medicine. That means less refined flour, sugar, and snacks and more vegetables, fruit, legumes, nuts, and seeds.

Strength training is critical for keeping metabolism humming as you age. Muscle burns more calories than fat does, explains Reardon, and without intervention, women start losing muscle as early as their mid-30s.

To boost your muscle mass, Michaels recommends strength training four times a week. It helps you burn more calories, even at rest.

Green tea contains antioxidants known as polyphenols that have mild metabolism-boosting properties, Reardon says. "Staying hydrated with a combination of green tea and water may counteract some of the metabolic slowdown that occurs with age."

Drink three to five cups of green tea a day for its calorie-burning potential, suggests Reardon, and at least 48 ounces of water to support the body's metabolic processes.

Portions have substantially increased over recent decades -- by some estimates, as much as 200 percent -- in both restaurants and at home. While it's fine to eat your fill of nonstarchy veggies, much of what we overindulge in (simple carbs, fats, meat) affects our weight and health.

Bagnulo suggests making sure half your meal is vegetable-based, a quarter is made up of healthy protein, and the final quarter consists of a whole grain or starch.

For desserts, try a smaller bowl. At an ice cream social thrown at Cornell University, those given a large bowl served themselves 31 percent more ice cream than those given a small one. Use a bowl that fits inside your two cupped hands and sip water before deciding if you want an additional helping.

Use chopsticks or a teaspoon, or hold your fork in your nondominant hand. "Anything that slows us down and brings the focus back to the food counters a tendency to overeat," says Reardon.

Stress, fatigue, and weight gain are common companions: Stress can lead to poor food choices; this can put your body on a blood-sugar roller coaster, which causes low energy. Sleep deprivation can cue your body to release stress hormones, triggering weight gain. And "the less time you have to devote to self-care," says Loring, "the harder it is to make the changes that lead to weight loss."

People who sleep less than eight hours a night typically have higher body mass than those who sleep a full eight hours, and babies who sleep less than 12 hours a day are more likely to be overweight by age 3. Although this don't prove that lack of sleep causes weight gain, an undeniable correlation exists between sleeping less and weighing more.

To sleep more -- at least seven hours -- avoid exercise, alcohol, and caffeine later in the day, take a bath before bed, and go to bed early.

Michaels uses a pyramid to help the time-strapped make goal-setting as effective as possible. Draw a pyramid on a piece of paper, and at the top, write down your ultimate goal in as much detail as possible -- total number of pounds to lose, yes, but also what the number will empower you to do (run a race, embark on a job search). For the next row, break that larger goal into monthly goals. From there, you can set weekly and daily goals.

"A mini meditation can provide the clarity of mind you need to make better food and exercise choices," says Loring. Sit in a way that supports an upright posture. Bring your awareness to the act of breathing for 20 breaths. When your mind wanders, which is normal, gently bring it back to simply breathing.

If you have unexplained fatigue and mysterious weight gain (no major changes to your diet or lifestyle) that has resisted your efforts, your thyroid hormones may be out of whack, says Richard Shames, M.D., coauthor of "Feeling Fat, Fuzzy, or Frazzled?" If you suspect thyroid issues, work with a doctor to get a proper diagnosis and treatment.

Although you eat well and exercise regularly, the spare weight you're carrying around won't budge. Michaels refers to the proverbial "last 5 pounds" as vanity weight: They don't affect your health, but they affect how you look. Whether your extra weight stems from having a baby (one study shows that moms tend to gain 5 pounds more than their childless peers) or simply becoming more sedentary, here's how to whittle it away.

To lose those last few pounds, Michaels says, keep your food intake steady and generate a calorie deficit through exercise. "You can either exercise more often or work out harder to push your body out of its comfort zone and melt those last few pounds," says Michaels. If you walk or run, add hills or stairs to your route and strength training to your regimen. If you primarily do the weight machines at the gym, start a spinning class. "Variety is key," she says.

"To jump-start your metabolism, eat breakfast 45 to 60 minutes after waking," says Reardon. "Keep it humming with a total of four or five small meals throughout the day." At each meal, get a balance of protein, slow-burning carbohydrates, and healthy fats, such as steel-cut oats with a teaspoon or two of almond butter and fresh berries for breakfast.

To make sure your efforts stem from a healthy impulse instead of outside pressure to look a certain way, Loring suggests regularly doing a loving-kindness meditation: Sit quietly and repeat a phrase such as "May I be healthy, may I be at peace, may I be content."

This practice helps you accept yourself as you are, says Loring, and guides your efforts so that they meet your true needs -- not a momentary impulse.

Following a rigid diet can seem easier than changing your eating habits, but the majority don't work in the long run and can lead to health problems. "Repeatedly going on very low-calorie diets carries a risk of permanently lowering metabolism," Reardon says. "Diets that eliminate entire categories of foods, such as carbs, can lead to vitamin deficiencies." Psychologically, constant dieting also breeds an unhealthy mind-set of denial and reward.

If you're looking for an eating plan to follow, consider the Mediterranean diet, which Reardon hails as "bar none, fabulous." Composed mainly of whole grains, fruits and vegetables, fish, olive oil, and nuts -- and low in unhealthy animal fats and refined food -- the Mediterranean diet "answers a lot of ills, including obesity, heart disease, cancer, and diabetes," says Reardon.

And there's more to it than food guidelines. "The lifestyle is a crucial component." Taking pleasure in preparing food and eating with family and friends adds to the nutritional benefits.

In addition to eating more fruits and vegetables, Reardon suggests adding a variety of whole grains to your regular rotation, including quinoa, amaranth, barley, millet, and spelt.

"They offer a wide array of powerful antioxidants, as well as fiber, which helps stabilize your blood sugar so your cravings for comfort food go down," she says.

Michaels says that the biggest obstacle to changing your habits is an all-or-nothing attitude. "It's a foregone conclusion that dietary lapses will happen. The trick is to see them as lessons and not as a confirmation that you're a failure." If you know you tend to eat too much cake at an office birthday party, for example, bring in a healthier snack of your own next time.