It sounds really dumb this but - "always make sure you have enough fuel". You don't want to spend the whole detail worrying about the fuel state or have to come when with another ten minutes flying the student would have grasped what it is you have spent the last hour trying to teach.
There have been at least two occasions when I have returned to the airfield and have had to orbit for 20 mins plus, and if I had not refuelled could have been seriously embarrassed.