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This brought me to mind of a conversation I once heard between Matt Chandler, Tim Keller and Michael Horton on this very issue. They lay out a few good guidelines for theological gentlemen's boxing:
1. When possible do it in the context of relationship.
Especially in the Internet age, it's easy to take people on from a distance. We all have a tendency (this is where original sin comes in) to paints people in bad and unfair lights. Whenever possible, before slamming a position get to know people that hold that position. This does mean you compromise your stance. It just means that you get to know the humanity behind the opposition.
2. Give the opposing argument in a way that your opponent would recognize and own.
Don't frame the other guy's position in a way that makes it sound silly or sinister. State in a way that he would gladly say, "Yeah, that's what I believe." Why? Because the alternative is fighitng a bunch of straw men. If the person isn't defining the terms the same way you are then you're really not wrestling with his ideas but your own misconception of his ideas. Also, this shows your opponent that you respect him or her enough to actually take the time to study their views and positions.
3. Be careful in assigning consequences to a person's beliefs.
It's one thing to say that a person's position will likely lead to something far worse. However, be careful not to imply that person necessarily holds to that far worse position. This is closely related to previous point. Get to know the other person's argument and see how they hold things in tension or how they might handle the slippery slope.
You can watch the whole conversation (which includes a nice bit about the Internet and reading) here: