Leave a note for the etacher: Leave a paragraph or 2 note to the teacher of the class's progress, and let it be known who was misbehaving along with any other notes (i.e. anyone dropped our of class, anything was broken, etc.)
Clean room before you leave: Pick up trash, put books in proper racks, erase chalkboards, return VCR, clean stains of walls, etc. While these are not part of your job, putting in extra effort never hurts to show your enthusiasm to teach and will help your chances of being brought back.
Grade Homework: It is good to grade homework if an answer key has be given. This is also not part of your job, but will leave a lasting impression and leave you higher on the list on substitutes. It also will give them absent teacher more time to do other things.