Holly at Education Choices offers some practical tips for substitute teachers. Among other things, she says, you should make sure you have notebook to write in (as a disciplinary tactic) and your:
Painkiller of choice (Aspirin, Advil, Tylenol, etc) - Students can be a pain, quite literally. The only thing worse than substitute teaching a bad class is substitute teaching a bad class while in pain.
You get the feeling she knows what she's talking about.
Hat tip: Creating Lifelong Learners