I know it's already been said, but it deserves repeating. Call your hits. Airsoft isn't like paintball, there's no splashes of paint to mark hits. No one knows for sure if you've been hit except for you. The whole sport rests on the honor of the players. So if there's any doubt, err on the side of caution and call the hit.
Another that deserves repeating, don't touch other people's equipment, especially guns, without permission. Some guns cost upwards of a thousand dollars or more and their owners are understandably protective of them.
One thing that I think is worth stessing but that no one is yet mentioned: don't fire your gun unless you're sure that everyone has eye protection on. It doesn't matter if you're firing in their direction or not, safety first.