The most effective way to prevent weight gain is through a combination of regular physical activity and a balanced diet. Engaging in at least 150 minutes of moderate-intensity exercise per week, along with strength training, helps burn calories while building muscle mass, which can boost metabolism. Additionally, focusing on whole, nutrient-dense foods while avoiding excessive processed foods and sugary beverages supports weight management. Monitoring portion sizes and maintaining a healthy balance between caloric intake and expenditure is crucial to prevent weight gain and promote long-term health.