Try this two-in-one abs-and-obliques move from David Barton Gym owner David Barton. Here’s how: Sit so thighs and upper torso form a V shape, with lower legs crossed and lifted.
Hold a 5-pound medicine ball (or dumbbell) between both hands. Swivel left to right and back, bringing ball across body while maintaining the V shape.
Do 3 sets of 15 reps 3–4 times a week.