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 ?
I used to argue with a more strident, arrogant tone than I try to adopt now. One influence in changing my tone was Ben Franklin's autobiography:
"I wish well-meaning, sensible men would not lessen their power of doing good by a positive, assuming manner, that seldom fails to disgust, tends to create opposition, and to defeat everyone of those purposes for which speech was given to us, to wit, giving or receiving information or pleasure. For, if you would inform, a positive and dogmatical manner in advancing your sentiments may provoke contradiction and prevent a candid attention."
He describes how he cultivated "the habit of expressing myself in terms of modest diffidence; never using, when I advanced any thing that may possibly be disputed, the words certainly, undoubtedly, or any others that give the air of positiveness to an opinion; but rather say, I conceive or apprehend a thing to be so and so; it appears to me, or I should think it so or so, for such and such reasons; or I imagine it to be so; or it is so, if I am not mistaken.
...
When another asserted something that I thought an error, I deny'd myself the pleasure of contradicting him abruptly, and of showing immediately some absurdity in his proposition; and in answering I began by observing that in certain cases or circumstances his opinion would be right, but in the present case there appear'd or seem'd to me some difference, etc. I soon found the advantage of this change in my manner; the conversations I engag'd in went on more pleasantly. The modest way in which I propos'd my opinions procur'd them a readier reception and less contradiction; I had less mortification when I was found to be in the wrong, and I more easily prevail'd with others to give up their mistakes and join with me when I happened to be in the right."
Another influence was Yvain's How To Not Lose An Argument. The common part of Franklin and Yvain's advice is to phrase your message in such a way that minimal status will be lost by your opponent agreeing with you. Your opponent must not see (consciously or subconsciously) your rhetoric as an attempt to gain status at zir expense.
To quote Daniele Vare: "Diplomacy is the art of letting someone have your way."