There's a simple bit of advice I've taken and tweaked from an old informercial that helps me to not make to much of an asshole of myself on the net.

Don't post angry.

If a post offends me or make my upset, I almost never post a reply right away. Instead I go and do something else for a while until I either can make a constructive response or let it go entirely.

The reason? I ask myself: What would you wish in return?