Some good suggestions here...

Most often people will disagree on issues of the day, but just because someone doesn't think like you doesn't mean you should call him evil and kill him. That's what Islamic extremists and the majority of posters at Democratic Underground would do. Instead, following these steps you can have a nice discussion with someone with different viewpoints and both come out of it smarter.

DO consider the merits of viewpoints different to your own.
DON'T throw out your own principles just to be agreeable. No one gains from that.

DO listen carefully to what the other person is saying. Try to understand their different viewpoint.
DON'T use the time while the other person is speaking as just an opportunity to formulate your own rebuttal.

DO try to find areas of agreement no matter how much you differ on an issue.
DON'T compare the other person's views to Hitler. Hitler was a bad man, and no one likes being compared to him.

DO keep on topic. You're more likely to learn from each other if you keep the talk focused.
DON'T kick the other person in the groin when he strays. He might forget what you were talking about in the first place.

DO make all disagreements in a friendly tone.
DON'T follow all disagreements with putting the other person's head through drywall. That gives who you're talking to no time to consider what you said. Also, it's hard to repair the drywall and paint over it perfectly to cover up the hole.

DO agree to disagree when reaching a stopping point.
DON'T declare an intifada and blow yourself and the other person up so you can get 72 virgins. You both lose if you do that, and you should focus on finding one nice girl (or guy) in this world.

DO stick to your principles while still considering what the other person says.
DON'T pile drive the other person into a folding table when you find a topic you vehemently disagree on. Though it would be cool, it's just not civil.

DO back up your statement with facts when necessary.
DON'T punch the other person through the chest, pull out his heart, and show it to him before he dies when you feel run into a corner. That's usually a non-sequitur to the debate... unless the debate is whether you can actually pull someone's heart out and show it to him before he dies.

There's more. The comments are funny, too.