When most people lift free weights, they usually place a weight in each hand to keep it nice and balanced. But health and fitness site Livestrong.com recommends that you just focus on working on just one side at a time, as the imbalance of weight can actually provide a better workout and exercise more than just the muscles you're currently targeting:
Since your core stabilizes your body, creating instability means it has to work that much harder. That means you can work your abs without ever doing a crunch. Here's how: Load one side of your body. Hold a weight on one shoulder during a lunge, press just one dumbbell overhead during a shoulder press, or perform a standing, single-arm cable chest press.