There are 3 things I now do to control my online behavior:
1. If I feel myself reacting badly to a message I've received and typing out of anger, I take my hands off the keyboard, scoot my chair away, and go knit or do something else relaxing until I have forgotten what I was angry about. I then come back to the message refreshed and, instead of looking at the original message, I go through MY message and edit it to sound the way I would want it to sound if I was receiving it.
2. If I feel myself reacting badly to a message from someone I don't know, I might ask for clarification on their meaning, telling them, "This is how I interpreted your message, but I want to be sure I understood it correctly because I often take things in a negative light when they aren't meant that way at all." Or, in the graphics Yahoo groups, I have a friend to whom I can send my outgoing messages, as well as the original messages, and she will act as my buffer between the public and my social ineptitudes. It has helped tremendously.
3. If I feel myself so near the boiling point that I am pouncing on everything everybody does and says, including my mother, whom I love more than anyone else in the world (I presently have no significant other), I will remove myself from everything fandom except for reading/viewing artwork and giving feedback, as I did rather recently.