Behave online as you would offline.
Remember that the people you are speaking to online are real people, with regular feelings and sensitivities. Anything that wouldn't be appropriate offline isn't appropriate online.
Be forgiving.
Everyone was a newbie once, even if it was back before that term was coined. If you see someone make a mistake, don't get upset. Can you ignore it? If so, you probably should. If not, point it out gently and give people the benefit of the doubt.
Help out.
Not everyone is tech-savvy, even if they've been using technology for a while. Many people have questions when they're online - feel free to answer them! Not only will the person you're helping out appreciate it but your answer will be cached by search engines so future questions can be answered with ease.
Remember there is a human in there.
You may be responding to a written paragraph of text, but it was composed by a person. Respond to them as if they are standing in front of you - no matter how old the post or email is that you're responding to.