I cook a lot of chicken breast. I've been buying fresh boneless/skinless out of the meat dept, but I've done the frozen bags plenty of times as well. Brining will help a lot. The ones out of the bags are usually pretty thin. If you don't want to brine, then I would suggest grilling hot and fast. If you find that there is a lot of difference in the thickness from the front to back of each piece, flattening them helps a lot with getting more uniform thickness and cook time. That way you don't have the super-dry-overcooked tips. Smoking works really well too, though. Chicken breast is pretty easy to tell by feel when it's done and you just want to get it as soon as it's done. It's pretty sensitive to overcooking, as many have already said. I also rub lightly with EVOO or canola, then sprinkle rub, or prepare a wet rub where I mix the rub into a paste with EVOO or canola and rub it on that way. Those steps also help lock some of the moisture in, I think.
Eddie G. - Smoke Bomb BBQ coming off a Lang 48 Hybrid Deluxe