#1, you're probably tired of people automatically believing everything they read on conspiracy sites, then posting it as fact, right? Fair enough, but don't you make the same mistake. Just because I might post an argument that agrees with what you believe, doesn't make me right. Do not read a page on here for the first time, then use it in a forum debate 5 minutes later: Think before you post.
Start by carefully reading what I'm saying (that is, not simply skipping over the arguments to the conclusion). Does my argument hold up, does it all make sense?
Next, look at an opposing site to see if I've covered the key points being made. You might read a page here saying there's no proof a particular hijacker is alive, for instance, and it may look convincing, but am I missing some key piece of evidence you'll find elsewhere? Do a quick Google to see exactly what the conspiracy sites are saying, and make sure I'm addressing whatever points they present.
Finally, do your own research into the topic. At a minimum, that means following the source links I've provided. I often edit quotes to extract just the relevant information, for instance, and while I try to ensure the original meaning of a piece isn't changed, mistakes could be made. Check the full article or whatever I'm referring to: does it still mean what I've said it means?
This may sound like a hassle, but it'll pay off in the long run. If I've made a mistake, or referred to a source link that no longer exists, say, whoever you're debating with will be quick to pick that up, and you'll lose the point. By doing your research first you greatly cut down the chances of being caught out by my error.
more:
http://www.911myths.com/html/arguing_the_case.html