Here are some definite diet no-nos upon which most diet specialists agree:

* Repeated cycles of weight loss followed by weight gain are occasionally dangerous and generally lead to fat stores that are increasingly difficult to lose. As one specialist put it: "When you diet, your body doesn't know you're dieting. It thinks you're starving to death." As a result, it mobilizies all its biochemical resources to protect those fat stores. This, speculates Georgetown Diet Center's Aaron Altschul, is probably why people, when they do regain lost weight, end up a few pounds heavier than they started.

* The worst diets are the ones that work the fastest. Weight lost quickly usually comes from lean body mass -- muscles -- not from fat stores. The body moves to restore muscle as soon as a normal diet is resumed. Among the worst offenders are high-protein diets, high-fat diets and extremely-low-calorie diets unless they are administered under the direct care of a specialist and contain carefully constituted ingredients.

* Exercise is crucial, and "hard, continuous, boring" exercise may be the only way for many to lose extra pounds. But ease into it, and be sure to check with a physician before embarking on a vigorous program. One theory holds that swimming is particularly good for dieters because cold water seems to stimulate thermogenesis -- burning off fat.