Organize the materials needed for each class before the students arrive. Students know when you are not prepared, and this will discredit your authority. If they get the impression that you do not know what you are doing, gaining and maintaining control will be very difficult.
Greet the students at the door to create authority and respect. Your students will also feel respected because you took the time to greet them, even if you will be their substitute only for one class period. A mutual level of understanding can be helpful in gaining control, because students will want to listen to your directions.
Know the established class and school rules, procedures and consequences in order to address and dismiss any attempts to break them. Being in the know will lead to fewer distractions and less time spent trying to verify students' requests.
Have additional activities prepared in case there are unforeseen problems with the lesson plan left by the classroom teacher. For example, the called-for activity may be too challenging for a lower-level class, and you might need to modify it. Or a few class clowns might even take the lesson materials and hide them. Being prepared for the worst-case scenarios will always give you the upper hand.
Use proximity by walking around the room to monitor student behavior. As a substitute, you will probably distribute the work and ask students to complete their tasks independently. If you fail to walk around and make your presence known, students will take advantage of their perceived freedom, which can lead to behavioral issues.
Give a warning and the consequence when a student does something against classroom or school rules. Students know the expectations with their regular teacher, and they should assume that yours will be similar. However, they will also expect you to be a little easier on them, so be up front with specific consequences to inappropriate behavior.
Follow through with consequences. If you warn a student that a continued behavior will lead to a detention, assign the detention the next time he or she breaks the rule. Otherwise, any control and respect you have gained as a substitute will be lost.
Address problems immediately before things get out of control. Students will want to test your authority and see what you allow them to get away with. If you ignore small things, they will expect to get away with large things, too.
Maintain your composure. Students might try to get under your skin and make you angry. As soon as you give in to yelling or other anger-driven actions, they will see a victory for them and continue to rile you. Take deep breaths if needed, and always think before you speak or do something when you feel angry.
Send disruptive students to the principal's office if warning them or redirecting them does not work. Other students will take note that you mean business.