1. Would you say it to someone in person? Yes? Really? If so, then go ahead and say it. If it is something you would even hesitate to say to someone in person, then WHY would you think it is okay to say it over the internet? If you can honestly ask yourself if you would say it to someone in person, then go and say it online. Otherwise, keep it to yourself.
2. Would you be hurt/upset if someone said what you are about to say to you? If the answer is yes, you better really think it over - because you might regret what you say. And if you have to say something and have to really get it out - then say it in person, because that way you wont be misinterpreted and things wont spiral into a big fight over some sort of comment.
3. That kind of ties into my third point. Can what you are saying be interpreted in a different way? In a more negative way? If it does, reword it. It is amazing how the tone of a sentence or paragraph can just with just a few changes. Be really carful.
I just really think that because we are becoming so dependant on cell phones (texting) and internet for communication, we forget how to communicate with others... and if we aren't careful, we can really lose that skill forever. Thats what it is, communication is a skill - and it is a really important one. But when you aren't communicating face to face it is really easy to say something that you would normally say, and become someone who you aren't.
Just, be a good person. Online and in person.