“Say unto others as you would have them say unto you.”
(I know someone said something like that sometime ago.)
Seriously, I fail occasionally to follow this rule and end up responding too quickly on blogs, but I have a rule to re-read what I have written prior to submitting it and make sure I wouldn't get ticked off if it was being said to me. I nearly always regret it when I don't follow the rule, even in situations where the conversation is flowing so quickly that waiting to re-read and edit causes my comment to appear a bit disjointed when it posts. Hurrying just isn't worth the mistakes I make when I do - both in wording and in tone.
When it becomes a true habit, it happens even with posts like this one - where I know there isn't anything hurtful in what I am typing. I have read this post more than once as I've edited each paragraph. It works for me - when I remember to do it. When I don't, I often end up saying something that doesn't need to be said.