I've not ever really had the problem of not thinking before I spoke. I've always tended to think about things a lot. It's just that I would think, and still say the wrong thing. But recently I've managed to think about all the things I could say, and then not say them. You know, like when someone who really should know better asks a particularly clueless question. (No, Beth, I'm not talking about you, not by a long shot.) I could say "RTFM" or something equally unhelpful, but I've learned how not to, which is really cool.
And eventually, later, I sometimes become really glad I didn't say mean things to people. For example, for a while I thought Beth and I rubbed each other the wrong way, but it turns out I was wrong, and she's a really cool person to have on my team. There's another woman who seemed annoying at the beginning of last quarter but whose personality has grown on me, especially since she knows everyone and apparently everything, at least within the iSchool. (I guess she's what they call a "gatekeeper.") So, yeah, not being mean pays off in the end.