The biggest way to ensure the chicken breasts stay moist is to not overcook them! The USDA advises that chicken breasts must be cooked to a temperature of 165 degrees F. So, that's what we are going with here; much beyond that, and you risk drying the chicken breasts out. For that reason, I recommend picking up an oven safe alarm thermometer as I recommended above - they're great because they will let you know exactly when the target temperature is reached. You don't have to keep checking the food constantly, or, even worse, risk the chicken breasts going way above the target temperature. Even if you do end up overcooking the chicken, just smother each bite in the delicious gravy that the chicken and vegetables are cooked in - this will really help to re-moisten the chicken and it will still be absolutely delicious!