7. Do boxing as part of your cardio exercise. This will work your core to flatten your belly. Throwing punches and twisting works your ab muscles well. Try about 15 min. 3 times per week of boxing to lose about 2 inches in your midsection within 4 weeks. Also punching while holding 1-2 lb. weights for 8 min. and then alternating arms and repeating without the weights faster for 8 more minutes will work great to get those flat abs.