"Eat what you want, when you want, and watch the pounds disappear!" You've heard of them, maybe even tried them: miraculous-sounding diets that claim to melt off pounds with minimal effort. There are hundreds of these quick-fix diets out there, from the grapefruit diet to the detox diet to the "caveman" diet. But how do you tell legitimate weight loss plans from diets that don't work (at least in the long run)?

One reason it's so hard to tell the difference is that even the worst diets will likely result in weight loss, at least initially. But it does little good to lose weight, experts say, if it comes right back.

"Don't be fooled into thinking it is because of some magical food, pill or potion. What causes weight loss is eating fewer calories than you burn," says Dawn Jackson-Blatner, RD, a spokesperson for the American Dietetic Association (ADA). "Crazy, unbalanced diets cause weight loss because they are basically low-calorie diets."

After a few weeks on an unrealistic diet, dieters usually become frustrated and give up. This leads to feelings of failure that can help send them right back to their unhealthy lifestyles.

"Fad diets not only fail to produce long-term weight loss, they can lead to deprivation, weight gain, and discouragement," says Michelle May, MD, author of Am I Hungry?What to Do When Diets Don't Work."In other words, you are often worse off than before you started."