Geese require more lead than you think. At 40 yards, you need 6-8 feet of lead on crossing shots. Practice leads on this range!
Mount your gun as the bird comes into range, then swing through the target. Don't stop your swing when you pull the trigger.
Know your effective range. Most hunters shoot at birds too far away. Pattern your gun to learn its limits - usually 35-40 yards max.
When multiple geese are working, pick ONE bird and focus on it. Sky busting at the flock rarely results in clean kills.
Let birds get close. Geese look bigger and closer than they are. Wait until you can see detail on the bird before shooting.
Steel, bismuth, or tungsten required by law. Steel requires 2 shot sizes larger than lead for same effect. BBB and BB are popular for geese.