Weighted Side Plank

Stack your feet and prop yourself on one forearm, elbow under your shoulder, lifting your hips so your body forms a straight line from head to feet. Hold a dumbbell against your top hip with your free hand, or extend that arm straight up toward the ceiling with the weight in it for a bigger challenge to your balance. Keep your hips pushed forward rather than letting them drift back, which is the easiest way to cheat the position. The added weight pulls you toward the floor on the working side, so your obliques have to work harder just to keep the line straight.