Flats are hard to keep from drying out. In another smoker, I'd say cook it in a pan with beef broth to keep moist. From what I have read on here, pans are frowned upon since they don't allow proper heat circulation by acting as a "shield". That's unfortunate if that indeed is the case.
Buy a packer if you can, you'll have better odds of making a moist brisket. Maybe Walt will chime in and give you a few tips. He knows far more than I do about briskets.