I'm not an instructor yet but hopefully my suggestions will be okay here...
#14: Always ensure that both sides of the cowling is clear before starting the engine. That means if you have someone in the right hand seat, ask them if they can check their area before you crank the starter...
#15: NEVER EVER EVER allow any of your students to advance or retard the throtlle quickly, ie a second gap from full power to idle doesn't help a normally aspirated engine, never mind a turbo charged engine. Ooooh always makes me cringe. A pet hate.
#16: Power checks with aircraft quite close behind isn't a good idea, especially if I'm trying to preflight the aircraft behind you!!!
#17: Checklists are good, but one can get over reliant on the printed checklists. One person that I fly with regularly, after taxiing doesn't put the parking brake on and reset the RPM to ground idle until he pulls out the checklist and reads that item...
#18: Correct Approach Speeds. Too many PPLs that I fly with fly their aircraft far too fast on approach (because that is the speed they were told to fly by the instructor) and then wonder why the aeroplane floats like a glider when they are trying to land... too much of that "add a knot or two for the kids in the back" seems to be prevailant.
#19: Circuits. Why oh why oh why oh why do we have to fly a circuit that is out of gliding distance to the airfield / airport? At my home base 2 mile final approaches are not uncommon with a mile or two out on downwind. Far too far out. Especially that most of our final on R30 is over water. Granted I know some fields you have to for noise abatement, but what is the excuse for the other airfields?
#20: Mixture. This seems to be quite a important item that is glossed over during PPL training here in the UK. In the USA it seems as though the instructors take a more active role into teaching what the mixture control actually does and how to use it correctly.
Apologies in advance to the instructors here, I thought I would just add some to the list above that you guys (and girls) have ommitted (but probably teach anyway).
I know I'm not perfect, far from it. However one day when I'm an instructor, I do hope that I remember to teach my future students what I preach here.
Best wishes,
Charlie Zulu.