Work on getting the arm straight in the air on every whistle. This is an important part of the mechanic for stopping the clock. Your hand should be an open fist for a violation and a closed fist for a foul. Keep working on this so it becomes automatic on every whistle.
As the lead official do not ball watch. This means that when the ball is being passed or dribbled on the perimeter trust your partner working as trail to officiate that action so you can focus off ball on screens, cutters, and post play. If both officials are watching the ball then there is no one watching the other action on the court. When observing new officials this is one of the first things a veteran official will watch for to see if the official has mastered good off ball coverage.