This move looks deceptively simple, but with the right amount of weight (grab a set of dumbbells that are challenging to raise over your head), your abs will be working overtime to help stabilize your spine with every step. Get even more abdominal recruitment using an uneven load: Grab a weight with your left hand that's about twice as heavy as the lighter weight in your right.
How to do it: Stand with your feet hip width, holding dumbbells overhead, palms facing in. Brace your abs in tight. Slowly start to walk forward, taking about 25 total steps (avoid letting the weights drift behind your head, and try not to overarch your back as you walk). Switch hands with your weights, and then walk back to starting position. That's one set. Try 3 sets total, taking a brief rest in between sets.