Know how to hand start your engine and carry out minor repairs.
FWIW, I posted this a while ago:
As with so many other areas of aviation, hand starting engines is one in which a detailed understanding of the specific fuel and ignition system fitted to the specific aircraft is essential for success and safety. ...
If your aircraft is fitted with e.g. an injected Conti and the only way to prime it is an electrical auxiliary pump, it's going to take an enormous amount of hand swinging to get that baby started if the battery's so dead that it won't run the aux pump to prime it. At hand swinging RPMs, the EDP isn't doing much at all. (The last time I hand started an IO520 the battery still had enough charge to run the aux pump slowly, but sufficiently to prime the injection system. [T]hey start easily when swung. Very - and dangerously - easily. But only when the F:A in the induction system and the spark are correct. Speaking of the spark....)
Some ignition systems use retard breakers in the magnetos and electric aids to starting, like "Shower of Sparks" vibrators and 'SlickStart' boosters. They have complicated wiring at and from the ignition switch. That wiring is different than if the magnetos each have just a mechanical impulse coupling. [Added: And some engines only have one magneto with a mechanical impulse coupling.] The electrically boosted ignition circuits are rarer these days - at least on 'traditional' GA piston engines - but not extinct. Trying to hand start one of those engines with a flat or low battery may be, at best, impossible, and at worst, deadly, no matter how competent you are at hand swinging in general. And note that the generic model of the same engine can have different magnetos and start systems. Not all e.g. IO520s have the same ignition systems.
In short: Deep systems knowledge of the specific aircraft you are flying is
essential.