One week you're stuffing your face with protein. The next week, a new diet urges you to dump the protein and load up with carbohydrates. Talk about redefining yo-yo dieting.

So do you continue with your current diet? Or will friends who have lost dozens of pounds on this new diet sway you over? All you know is that you're desperate to lose weight. If that new diet works, then it's worth a try.

Before you dig into another diet, step back and evaluate it. Just as you wouldn't buy a car without knowing anything about it, you shouldn't jump into a diet without scrutinizing its claims. And before you continue your string of yo-yo dieting, you should learn what successful weight loss is all about.

The Keys to Successful Weight Loss

Weight loss doesn't happen overnight. Nor should it happen to the tune of 10 pounds a week. Instead, successful weight loss means losing 1-2 pounds per week, says Debra Wein, MS, RD, sports nutritionist and co-founder of the Sensible Nutrition Connection in Hingham, Massachusetts. "When you lose more than 1-2 pounds per week, you lose more than just fat mass," Wein says. Instead, you start losing part of your lean body mass, including muscle—the mainstay of your metabolism. Muscle, after all, uses more calories than fat and is a major contributor to helping increase metabolism.

There are 3,500 calories in a pound, so to lose one pound a week you need a 500 calorie deficit per day, which is ideally achieved both by cutting back on calories and through exercise. Regular exercise is an important component of weight loss success. "You need to get a balance between caloric intake and caloric expenditure," Wein says.

For weight loss to be successful, you also have to incorporate exercise into your daily routine. "You need to get a balance between caloric intake and caloric expenditure," Wein says.

You may also need to change some of your eating patterns, for example, are you always eating in front of the television without realizing how much you've eaten? Do you eat when you're depressed, sad, or angry? Ideally you should pinpoint what triggers you to eat, and if it’s not hunger, develop new, more healthful, responses to these triggers.

"You need to analyze your eating patterns and develop new responses to old stimuli," says Rick, who readjusted his behaviors in 1986 and has since maintained a 120-plus pound weight loss.

Why Diets Fail

Inevitably, diets do—and most likely will—fail. Consider, after all, how many times you've been in this situation. You go gangbusters on one diet only to fizzle out after a few weeks. Then slowly but surely, the weight you've lost creeps back onto your body. What went wrong?

Research backs this point. A study of four popular weight loss diets—the Ornish, Zone, Atkins, and Weight Watchers—found that all led to modest weight loss when followed. But sticking with the diets wasn’t easy, with 42% of dieters dropping out before the end of the study. Diets often don't work because they're simply temporary interventions and don’t address the issues really at hand: what is causing a person to eat a certain way and why? " Most diets, for instance, prescribe certain eating habits that you follow for a specific period. Yet once that period ends, you're left to battle with your old eating patterns. Although you may have lost weight, you didn't learn anything about nutrition, nor were you taught how to modify your old eating habits to maintain the weight you've achieved.

Many diets are also too restrictive or unrealistic, Wein says. "Someone may be eating so few calories that they just can't function well," she explains. Or the diet may require giving up going out to eat with friends or even eating certain food groups.

How to Spot a Healthy Diet

So how can you choose a diet that will help you lose weight sensibly and keep it off? By taking the time to evaluate diets and not believing every claim you read or hear. Before you start a diet, Rick advocates talking to your doctor about your intentions. Then ask these questions when analyzing a diet:

1. Who is the author of the diet?
Make sure that the author has credentials to back his or her expertise, Rick says. Even if a diet book is written by a doctor, find out that doctor's area of interest and look for motivating factors that might have prompted him or her to write the book.

2. Are the diet's claims backed by research?
Do some digging to find out whether research has been performed, preferably at the university level, Wein says.

3. What are the health risks associated with this diet?
Make sure a diet lists the health risks involved—even the most sensible diets can be risky for certain groups of people, such as women who are pregnant or people with diabetes. If a diet seems too good to be true, it probably is.

4. Are all food groups represented in the diet?
"If you cut out a whole food group," Wein says, "you miss out on valuable nutrients." Without the right balance of nutrients, you'll feel sluggish and will perform poorly throughout the day.

Also keep in mind that the key to losing weight is to cut back on calories, not to focus on a particular nutrient. Researchers at Harvard School of Public Health studied four groups of dieters; each group followed a diet that emphasized different nutrients (eg, a low-fat/low protein/high-carb diet). Weight loss was the same for all of the dieters, no matter which diet they followed. The key to weight loss was sticking with whichever diet they were assigned to.

5. Does the diet severely restrict calories?
Severe caloric restriction should send up a red flag, Wein says. Women who are exercising regularly require a minimum of 1,600 daily calories, she says. Active men require at least 2,000 calories every day.

6. Does the diet recommend something other than high carbohydrate, moderate protein, and low fat intake?
If so, consider finding another diet. Carbohydrates have received a bad rap lately for no reason, Wein says. "Our bodies and brain rely on carbohydrates for energy," she says. Take in too few carbohydrates and you'll feel drained. On the other hand, eat too much protein and you'll wind up fighting dehydration, exhaustion, calcium excretion, and other health problems.

7. Does the diet claim that weight loss will be immediate?
Remember that slow and steady sheds the weight; focus on losing only one to two pounds per week.

8. Does the diet reveal how many pounds the average person loses?
Before and after photographs can be enticing but deceiving. The weight loss industry is largely unregulated and thus appealing photographs don't represent average weight loss.

9. Does the diet encourage exercise?
Exercise is a vital part of weight loss and management and should at least be recommended.

10. Does the diet propose a maintenance plan once you've lost weight?
Losing weight is easy for many people, but developing a plan to keep the weight off is key to long term success. This is why extreme or radical diets generally don’t work—they just aren’t maintainable in the long run.

A Life Change, Not a Quick Fix

As Wein is quick to point out, there is no quick fix and no magic pill or supplement that will make you lose weight. ""The bottom line is that to lose weight," she says, "you have to eat well and exercise."

Once you do that, you're on your way to a lifetime of weight management, not yo-yo dieting. "Just like riding a bike," Rick says, "the longer you manage your weight, the easier it becomes."