I've filed three (about one every decade).
The closest call I had was actually during takeoff.
A glider pilot landed his glider over my aircraft (Bulldog) while my student was flying our takeoff, just before lift off. I took control and avoided his glider by the narrowest of margins, my starboard wing went over his port wingtip. I had no other option because if I'd tried to stop we would have run up the back of him.
Lookout is the best answer (and to a lesser extent listen out on the correct frequency to help build a mental picture).