Lots of great suggestions here. A few of mine:
Customer service/retention:
-Front desk staff that is warm and friendly. This is huge.
-Introduce students to each other, invite them to back seat each other's flights.
-Have the students try different instructors, especially if they are plateauing at all.
-Make sure the more advanced students know that they are an example to the newer students and should make an effort to be responsible/welcoming/helpful.
-Punctuality.
-No hidden costs. Everyone knows flight training is going to be expensive. If you feel nickel and dimed though, it can be very off putting.
-Encourage them to take photographs and video. If they share their experience with their friends they'll be less likely to quit.
Instructing points:
-Debriefing after every flight.
-Clean windscreens. Never saw anyone clean a windscreen at my US flight school, at my NZ one it was part of every preflight.
-Experience actually using different airports, not just taxi back and take off. Refueling, using an FBO, finding parking, these are things I had to learn on my own, when they could have easily been shown to me by my instructor during cross country flights. (This can be filed under "teach to fly, not just pass the checkride")
-More focus on weather decisions. These are the hardest decisions to make as a fresh pilot, they need lots of focus. Have your students interpret the weather every day, make their decision, then call an experienced instructor to talk about their interpretation.
-During flights, ask your students what they would do if they were running out of fuel, if a passenger was sick, started losing RPM, a door is open on take-off, low voltage light goes on, find broken equipment at a foreign airport, etc.
These things will leave your fresh PPLs safer and more confident, which means they'll enjoy flying much more.