Effective communication techniques, particularly in written communication, are an important part of the aspiring rationalist's toolkit. Alicorn's recent post makes excellent points about niceness, and touches parenthetically on the larger issue of form versus content.
The general claim, when defending either rudeness or poor spelling, is "what matters is the content in what I'm saying, not the form". Well, I suspect this is one of the myths of pure reason. What matters about your content is what you do with it, pragmatically. Are you here to convey ideas to others ? Then you will achieve your aims more effectively if nothing about the form distracts from the content. (That you need to have content goes almost without saying.)
Conscientious programmers are aware that source code is read and modified much more often than it is written. They know that it's harder to debug code than it was to write it in the first place. They invest more effort in making their code readable than a naive programmer might, because they estimate that this effort will be handsomely repaid in future savings.
Conversation is no different. Your intent (in a forum like LW, anyway) is to cause others to ponder certain ideas. It's in your interest to consider the limitations of your interlocutors, their expectations, their attention span, their sensitivity, their bounded rationality, so that the largest possible fraction of your effort goes into delivering the payload, versus dissipating as waste heat. There are more readers than writers, making it rational to spend time and effort working on the form of your message as well as the content.
You even need to keep in mind that people are stateful. That is, they don't just consider the local form you've chosen for your ideas; they also apply heuristics based on past interactions with you.
These considerations apply to more than just "niceness". They apply to any instances where you notice that people fail to take away the intended message from your writings. When people respond to what you write, even with criticism, a downvote or a complaint, they are doing you a service; you can at least use that feedback to improve. Most will simply ignore you, quietly. Given enough feedback, the form your communication will improve, over time.
And I would be quite surprised, given what I know of human minds, if this did not also eventually improve the content of your thinking. I find exchange with others indispensable in sharpening my own skills, at any rate, and that is why I aspire to be not just nice but also clear, engaging, and so on.
I have gotten a lot of mileage out of, among others, Richard Gabriel's Writer's Workshop book, and Peter Elbow's Writing with Power which introduced me to freewriting.
What techniques do you, as rationalists, find useful for effective communication ?