Overcooking is usually the reason for dry chicken, so cooking it on low is the best option, particularly if you're working with a new slow cooker and you're not familiar if it runs on the hotter or cooler side. Your chicken will also come out dry if you use a slow cooker that's too large. For this recipe, a 5-6 quart slow cooker works just fine. Finally, adding liquid helps keep the chicken moist, though the chicken will also release some of its own juices while cooking. For this recipe, the salsa provides the needed moisture.