How does behavior modification help you maintain healthy body weight?

Rewards appropriate. Behavior modification is an approach to changing behavior through stimuli to act and reinforcement or lack of reinforcement of actions toward the stimuli or situation (s-r-reward). Principles are demonstrated in one of the classic books on weight loss, "slim chance in a fat world".
In complex ways. Maintaining body weight is an elaborate dance between the competing factors of genetics, hormones, environmental cues, food components, eating habits, and exercise. Behavior modification is complex, and effective recommendations are tailored to your specific situation. Common modifications include regular meals of diverse foods, lots of plants, few starches/sugars. Exercise 20 minutes 3x per week.