Have an argument. Once you start an argument, not a discussion, you’ve already lost. Think about it: have you ever changed your mind because someone online started yelling at you? They might get you to shut up, but it’s unlikely they’ve actually changed your opinion. Forget the pitfalls of Godwin’s law. Any time you mention Hitler or even Communist China or Bill O’Reilly, you’ve lost. Use faulty analogies. If someone is trying to make a point about, say, health care, try to make an analogy to something conceptually unrelated, like the space shuttle program, and you’ve lost. Question motives. The best way to get someone annoyed and then have them ignore you is to bypass any thoughtful discussion of facts and instead question what’s in it for the person on the other end. Make assumptions about their motivations and lose their respect. Act anonymously. What are the chances that heckled comments from the bleachers will have an impact? Threaten to take action in another venue. Insist that this will come back to haunt the other person. Guarantee you will spread the word or stop purchasing. Bring up the slippery slope. Actually, the slope isn’t that slippery. People don’t end up marrying dogs, becoming cannibals or harvesting organs because of changes in organization, technology or law. Go to the edges. This is a variant of the slippery slope, in which you bring up extremes at either end of whatever spectrum is being discussed.
Seth makes some good points here. In addition, I suggest if you want to Win an Argument online treat it more as a debate than an argument. Bring evidence to the table to support your point of view. Ask clarifying questions for further understanding. Keep a positive attitude and challenge your opponent to explain their position rather than shoot them down entirely. Consider the issue from their point of view a predict their rebuttal.