Nothing beats aerobic exercise in the battle against belly fat.  A study by Duke University found that aerobic exercise was the most effective way to burn deep, visceral belly fat and that aerobic workouts burn 67% more calories than resistance training or a regimen that combines cardio and resistance. The Department of Health and Human Services recommends that healthy adults get at least 150 minutes a week of moderate aerobic activity (think brisk walking or swimming) or 75 minutes a week of vigorous aerobic activity (such as running) spread throughout the week. Activity sessions can broken up throughout the day but should be at least 10 minutes long.  If you're looking to lose weight, you'll have to increase the intensity, the duration or both. There may never be agreement among trainers as to whether or not crunches are the best way to flatten your abs, but there's no disputing the fact that they work the muscles on the front and side of your abdomen.   Pelvic Tilt Crunch. Lie face-up on a stability ball with your back and head pressed into the ball, your feet together on the floor and a 5- to 10-pound dumbbell or medicine ball in your hands positioned against your chest.  Tighten your abs and crunch up until your shoulders are off the ball. Then use both hands to reach the dumbbell or the medicine ball up toward the ceiling.  Do three sets of 12-15 reps resting for 30 seconds between each set.  Arms Over Straight-Leg Crunch. Grab a pair of 10- to 12-pound dumbbells and lie on your back with your arms behind you and your legs extended and raised to a 45-degree angle. Bring your arms up over your chest and lift your shoulders off the mat while raising your legs until they're perpendicular to the floor. Return to your starting position without letting your legs touch the floor. Do three sets of 15 repetitions with a 30-second rest period between sets. Your core is made up of your abdominal muscles plus the muscles of your lower back, pelvic floor and hips--more than 15 muscles in all.  For a really flat stomach, you need exercises that target all of these muscles.   Side Plank. Lie on your left side with your elbow directly beneath your shoulder and your legs stacked one on top of the other. Place your right hand on your left shoulder or on your right hip.  Tighten your abs and lift your hips off the floor until you're balancing on your forearm and feet so that your body forms a diagonal line. Hold for 30 to 45 seconds. Repeat on the opposite side. If you can’t hold the pose for 30 to 45 seconds, stay up as long as you can and work you way up.   Push-up Walkout. Get on the floor in a push-up position and place your hands so they're two inches wider than your shoulders.  Keeping your feet in place, walk your hands out as far as possible, then walk back. Do 10-12 reps. For more of challenge, you can lift one leg before you walk your hands out and back.   Climbing Rope. Sit down with your legs extended out in front of you and your feet turned out in a V position.  Point your toes.  Contract your core muscles and roll your spine into a C-curve. Lift your arms up and move them as if you were climbing a rope twisting slightly with each reach. Do 20 reaches with each arm.

Summary:
Go with cardio. Do your crunches. Work your core.