Always aim ahead of flying birds. The faster they fly, the more lead you need. At 40 yards, lead a crossing mallard by 4-6 feet.
Keep your swing going after pulling the trigger. Stopping your swing is the #1 cause of missed shots on waterfowl.
In a flock, focus on one bird. "Flock shooting" rarely works. Pick the trailing bird—it's often a drake and easier to track.
When ducks cup their wings to land, they slow dramatically. Reduce your lead. This is when most hunters shoot behind.