Inform students that any misbehavior will be reported to their regular teacher. To deter class clowns from acting up, tell them their actions will have consequences, even if you aren't the one to administer them.
Avoid showing too much anger in front of problematic students. Certain students misbehave for the sole purpose of eliciting overblown reactions from teachers, so make a point of not giving them what they want. When it comes to bad apples, be firm, but abstain from raising your voice and making empty threats.
Follow the regular teacher's lesson plan. This will give the students a sense of structure. Avoid turning the class period into a glorified study hall, as class clowns are more likely to cause trouble if they have nothing else to occupy their time.
Offer your class positive comments whenever appropriate. For example, if the class you're left in charge of does a good job of being respectful and staying busy, let them know how much you appreciate their cooperation and promise to leave a favorable report for their teacher.