My preferred way to grill chicken breasts is to light up the coals, and once they've ready to move them all to one side of the grill. I will cover the chicken in olive oil, thyme, and season salt, and will sear them directly over the coals on each side for around 2 minutes, or until you have nice grill marks. Doing that will help hold in a lot of the moisture and flavor, but gives a good flavor to the outside. After that I will move the chicken to the opposite side of the grill away from direct heat, and cook until it's about 165F in the middle, which usually takes around 20 minutes. You'll have a fantastic smokey flavor, and very tender chicken.