Before you cook the meat, let it get almost room temp. If you start with frozen or very cold meat it will take a lot longer for the center to come up to temp. This will cause it to be tough. Fall apart= low and slow, also keep moist. Moisture in the smoker or wrap it part way through the process. Wrapping and putting in a cooler lets it keep coking and cool off slowly. This lets the juices go back into the meat.