Digesting high-fiber green vegetables is an easy way to make your metabolism work harder, and it can kill cravings for unhealthy foods. Likewise, your body burns twice as many calories by digesting proteins than it does when digesting carbs. Add more protein to your diet with two fish-based meals per week. Packed with protein, fish, such as halibut or salmon, also contains omega-3 fatty acids, which increase fat-burning enzymes in the body.