I sub for elementary grades. It's funny, I find the younger ones easier!
First of all, most teachers already have a discipline method already in place: clothespins by their name, name on board, etc. You can always ask a neighboring teacher what they normally do. I always try to befriend the teacher next door!
If there is no discipline method in place, you need to commit to one and nicely, but clearly explain it, and your expectations (raise your hand to talk, stay at your desk, hands to yourself are usually the 3 main things) to the kids when you first begin the class. Also, make sure they know that you will be leaving a note for their teacher telling her who acted up and who was good:
1) Name on board: 1st infraction - name on board, 2nd - X by name means no recess or at our school they walk laps during recess, 3rd - remove from room (this can be to another teacher's room or the principal although I hesitate to send there unless it was really bad) or lose recess the next day (yes, you can do that)
2) along with the discipline plan, have a positive praise plan for those who are good so you're recognizing both good and bad behavior. When someone raises their hand before talking or whatever, say Thank you for raising your hands and following the rules. Offer extended recess to those who don't get in trouble. Let them play a fun game if they behave for a certain period of time. Bring in a bag of goofy toys (dollar store) and let them choose one at the end of the day for good behavior. Offer incentives for positive behavior!
Just make sure they know up front what is expected and what the consequences are (good and bad).