Researchers conducted a study in which women were given a first course of a large portion (three cups) of low-energy-dense salad. The salad was made with greens, vegetables, nonfat Italian dressing and reduced-fat cheese. Following that, the participants ate a main course of pasta.
Eating the salad boosted the women's feelings of fullness and reduced their total meal calorie intake. In other studies, having a first-course soup instead of the salad produced similar results.
Why does this work? You get an awful lot of food without many calories, which then helps to displace the calories in the next course of higher energy-dense foods. Simply drinking more water doesn't have the same effect.