Victory at any cost!
When arguing on the internet, it's very easy to lose track of where you are and what's actually happening and start to think that you're actually having a productive debate or something. This is a trap that must be avoided. The only goal is victory, at all costs. It doesn't matter if you're right. It doesn't even matter if you realise you're wrong half way through. Stick to your guns and win dammit! Here's how:
1: Attack your opponent.
Remember, you don't have to be right to win. It's just as good to make your opponent lose, because then you become the winner by default, and there's no better way to make someone lose than by direct attack. It saves a lot of time if you can skip over the whole "Your argument is flawed because ... and therefore you are wrong." and get straight to the "You previously said ... and therefore everything you say is stupid, so your a clearly wrong now."
You don't even have to know your opponent to do this. Just make wild assumptions about them and act as though they're true. Never be too specific, and always assume that everyone else will back you up. They probably will. If it sounds like it's common knowledge, no one wants to admit to being out of the loop.
If you feel that making things up about your opponent directly may be difficult, try to associate them with a group and then make up things about the group. "You say X. Group A says X. Your are therefore a member of group A and we all know that group A are a bunch of liars who base their arguments on lies."
2: Every valid point has a careless phrase.
If you seem to be losing, it's vital to pay extra attention to exactly what your opponent is saying. Pick up on the first careless phrase you see. Point out how stupid it is. Pretend it's the central tenet of your opponent's argument. Pick at it incessantly. Make sure your opponent can't get the debate past this point. If they rephrase to clear up any ambiguity or correct any slips of the mind, accuse them of changing their stance mid-argument.
3: Change your stance mid-argument.
Don't just suddenly switch, be a little subtle about it. Gradually work your way around to a different line of attack, or even a different topic entirely if you like. An argument about why gun control is bad can easily transform into one about how American laws are stupid and from there into one about which nation's people are stupider and from there onward to pretty much anything you want. Keep it moving, in any direction, until you get to something you can win.
Feel free to change the topic more than once, but not too often. People will start to notice. This can be counteracted by claiming that the topic drifted naturally, or (if you've only changed stances, not topics) that your opponent has been misinterpreting you from the beginning, but if it happens too often it becomes too obvious and you may be in danger of getting drawn back into the original argument.
4: Make unfounded assumptions about the argument.
It doesn't matter what your opponent is actually arguing in favour of, all you need to do is make it appear that he's wrong, because if he's wrong, you must be right, therefore you win. So before it becomes too obvious what your opponent's point actually is, leap to conclusions and then argue against those.
5: Use your opponent's words against them.
Keep logs of the conversation handy, so that if your opponent tries to point out that they weren't saying what you claim they are, you can pull some quotes out of context to make them look like they're lying. Also, keep an eye out for things your opponent says as you go that might work against him later. In a real-time discussion, it's important to be able to get your responses out in a timely manner, so it's good to remember who said what when.
6: Plead ignorance.
Remember, your opponent has no way of knowing what you know, so if they cite something that helps their case, but which is not readily available, claim that you've never read it and can't get hold of it on such short notice, so they'll have to provide some alternate source.
7: Plead stupidity.
If your opponent makes a valid argument, but it doesn't seem to have been generally picked up by the spectators, simply claim you can't understand it. Make them rephrase it or expand upon it. This gives you more time to think and also more opportunities for your opponent to make a mistake or for your to change the topic or your stance.
8: Make shit up.
Pretend whatever argument you're making has popular support, or the support of experts. It doesn't matter if it does or not, because it's pretty much impossible for your opponent to check. "I read an article that said..." or "[Expert] has said..." are great phrases to use because there's no way to check. Where did you read it? Oh, you can't remember right off the top of your head, so they'll have to take your word for it.
9: Ask them to explain their position.
Pretend you can't understand the source of their belief. Ask them to explain why they believe what they do. This should give you plenty more material to work with and pick holes in.
10: Deliberately misinterpret.
Take advantage of any ambiguity in whatever your opponent says by assuming the meaning that's easiest to argue with, even if it's clear that that's not what they meant. If they point out that that's not what they meant, accuse them of changing their stance mid-argument.
11: Pretend you already had this discussion.
It's much easier to win an argument you already one once before, so if you're arguing with someone you talk to regularly, just "remind" them of the "last time" you had this argument and that you won it then too.
12: Accuse your opponent of not paying attention.
If your opponent points out some previously unseen hole in your argument, and the discussion has been going for some time, the simplest method for dealing with it is simply to claim you already did. People don't want to go digging through logs for something they don't believe is there in the first place, so if they accuse you of lying, just tell them to check the logs. If they do (or claim they have) then accuse them of deliberately skipping that bit so as to avoid looking foolish.
13: Pretend you've already won.
Immediatly after answering one of your opponent's objections to your argument (even if you did a half-arsed job of it), pretend that you've answered all their objections. Ask them if there's anything else they weren't clear on. Act like you've made it absolutely clear to all concerned that your position is the correct one. If you pull this off, you can immediately follow it by appealing to the crowd to vindicate your victory.
14: Appeal to the crowd to vindicate your victory.
If at any point you seem to have won the support of a large portion of the spectators, simply point this out to your opponent. Accuse them of missing something that everyone else managed to pick up on. Ask the spectators to join in and help convince your opponent of how right you are. At this point you don't even really need to carry on, others will do it for you. And if they eventually lose, well, you stopped arguing ages ago, so you never really lost. You just left the debate for a while and will be happy to pick it back up at any time...
15: Point out whenever your opponent does any of these things.
Take the moral high ground and point out every mistake or dirty trick your opponent uses. It doesn't matter if they used a logical fallacy because they were careless or because they were intentionally trying to win. They must not get away with it.