My first assistant-coaching job in football was at William & Mary in 1961. The pay wasn't much, so to get $300 more per year, I agreed to coach the golf team. I didn't even know how to keep score, and really, my main job was not to wreck the van on the way to tournaments.
You always have to prepare for the obstacles that are going to come. Consequently, when they do come, it doesn't affect you mentally near as much as when you're unprepared for them.
I'd say handling people is the most important thing you can do as a coach. I've found every time I've gotten into trouble with a player, it's because I wasn't talking to him enough.