I used to teach elementary for 3 years. The biggest piece of advice is all about body language. Make lots of eye-contact with the kids to show them you aren't nervous or intimidated but friendly and easy to talk to. Smile and enjoy their company. That's outside of class. Inside of class, let the students know you can be stern when it's time to get down to business.
Also, take their things and pretend that you are about to eat it! Pencils, books, cel-phones! It's a riot! Their reactions are so cute!