Another stopgap measure which has helped me is to, when I finish a statement (maybe finish writing an email, or finish making a point in conversation) consider whether I've been unreasonable. An immediate correction can follow in person, or for an email, a new revision.

• Will the person receive what I've said well?
• Did I think about how they might react and shape my statement to make them react well, or did I just say things I wanted to say without consideration?
• Has my point been made in good faith, or am I searching for justifications?
• What do I really believe about what I just said?

(Some of that has more to do with being rational than reasonable, but the two aren't completely different, after all.)