Going back to the point about rebellious players, I know from personal experience that there can be quite a large difference between when you start noticing a player's petulance, worrying about escalation and even feeling intimidated by him and the point at which he must incur a caution. Even if it isn't serious enough to affect match control, it is like a constant, unpleasant noise in your head that can have a knock-on effect on decisions. Not being afraid to bring out a yellow as the context befits is one thing, but since we are not there to look for cautions, what is working for me is plain old experience. Referee enough 'tough' games to build up your tolerance to tension, and you'll probably find you'll be able to deal more calmly in interactions with players about dissent and they'll have less nervousness to feed off.