High-fiber foods generally take longer to chew. This gives your body time to register when you're no longer hungry, so you're less likely to overeat.
And high-fiber foods tend to make you feel full longer, so you won't be as ravenous later.
Finally, high-fiber foods tend to have fewer calories for the same volume of food.
Choose whole grains and whole-grain products, fruits and vegetables, beans, and peas and other legumes.