Let kids do as much as they can on their own, don't take things out of their hands unless asked.
Don't touch kids, even in seemingly harmless ways (hugs and pats on the head can even lead to trouble)
Know specific class rules and how to regain control of a class as well as class jargon (For example, calling a specific sitting position “criss cross applesauce” instead of “Indian style”)
Know the class signal for silence. This can be a hand gesture or a clapping chant. Using the wrong signal will confuse the kids.
Always bring hand outs or give away items. Kids ALWAYS want to take something home.
Never leave anything unsafe unattended, kids will touch it. They want to eat anything and will touch/take/stick fingers in anything they can.
Expect and be able to handle strange and unexpected questions or statements (“I have a hamster named Blinky” when you asked about the inventor of the lightbulb).
Above all roll with the punches - kids can be an odd bunch.
Be patient and willing to repeat yourself.
Try not to move the kids from one space to another. It takes time to control them and have them refocus. If you have inside and outside demonstrations, do the inside portion first and then bring the kids outside. It is very difficult to have kids settle down from an exciting outside demonstration.