Get a good rbf/intimidation factor if that suits you (maybe yours is a charm/sweet factor). Being personable while still being in charge of things helps a great deal too. You know, it's okay to smile and stuff when students are clever or funny. Also, best to be stricter first/early then you can be nicer later. If you let them get away with things, they'll get away with things. Gotta nip that shit in the bud early, you know, don't tolerate it. Or, if it's a class that you know won't listen at all, just document their names (hopefully you have a photo roster) for their regular teacher to deal with. If things are really bad, definitely call for backup. But really, bad classes are not on a guest teacher. If you have a bad experience with a teacher, or multiple bad experiences at a school, probably best to try for jobs at other schools until you find one that works for you.