|Because these days every blog post needs an image.|
You can help someone understand but you can't make them agree. Remembering this takes a lot of the stress out of a debate for me. Of course I'd like my opponent to agree with me but that's probably not going to happen during the debate. Throwing out that goal really helps me stay calm. I can make sure they understand my argument but I can't make them agree.
Remember the people who are reading but not participating. This is my mantra while debating online. I usually assume that my opponent is a lost cause so when I respond I'm really speaking to the silent bystanders. This helps me address my opponent's arguments without getting heated.
No personal attacks and no correcting grammar or spelling. As soon as you deviate from the topic to attack something unrelated, you've lost. I always assume that when my opponent points out a spelling error they are out of relevant responses. Pointing out grammar and spelling should be saved for when you're making fun of your opponent in a private group (what? You know you do it).
Obviously, you need to know what you're talking about. I like to provide links to mainstream-friendly articles which back me up. If you're debating with an atheist, you wouldn't use links from a Catholic website, right? No, you'd use secular links. By the same token (my dad always uses that phrase), if you're debating circumcision you shouldn't link to an anti-circ site at first. There are articles on Yahoo! News and The Washington Post that make your point, and they would be better received than a link from an anti-circ site. Now, if someone is genuinely interested in learning more about genital mutilation, then of course send them to The WHOLE Network! But if they're currently in favor of genital mutilation then sending them to a site that is against it will be ineffective.
Type what you want to say then delete it and type what you should say. Get that venting out of the way so your emotion doesn't overshadow your argument. Or yell at the computer, take a breath, and calmly compose a response - just don't go off on your opponent. For a debate to be effective it should be a discussion, not a fight.
And a quick message to silent bystanders - If an online debate has made you think or convinced you to change your mind, please let the debater know! Hearing that I've made a difference for even one person is what keeps me going. There have been several times where I have been on the verge of burning out when suddenly I receive a private message thanking me for what I've said in some debate. It gives me a second wind and I can't express how much I appreciate those messages. So if you like what someone says, let 'em know!