5. Cross-Behind Lunges
This workout is great not only for your abdomen, but for your legs, butt, and shoulders as well.
How to do it: Stand straight with your feet shoulder-width apart and your arms extended at the sides, holding one dumbbell in each hand. Then, step forward and to the side, so that your right leg is positioned behind your left leg. Gently lower your body until your front knee (left knee) is bent at nearly 90 degrees. Press into your left foot to go back to the starting position. Repeat the same movement on the other side.