It may sound the most blindingly obvious thing but one of the best pieces of advice I got when I started a year ago was "always make sure you have enough fuel".
There have been three or four occasions where I might have gone with what I had but remembered those words and topped it up only to be very glad that I did.
Had to orbit away from the airfield recently for 20 mins due IFR traffic and a jet doing circuits at the end of a trial lesson. Had I not topped her up I might have been seriously embarrased.
I tell my students they should enjoy flying and I should enjoy flying with them. If one of us is not enjoying it there is a problem.