They say you shouldn't talk about politics, religion, or money with friends.
Naturally, I violate all 3 rules regularly. I have some friends I've learned not to discuss politics with while there are others I can heartily talk about politics. Here are some of the rules I've come up with over the years.
#1 DO NOT discuss politics with people who feel that a political disagreement is tantamount to a moral failing on your part. There are people out there who believe that a disagreement in political philosphy is the result of you being a bad person, morally corrupt, etc.
#2 DO NOT discuss politics with people who feel that political disagreements with them rise from ignorance on your part. Some people become so unempathic when it comes to politics that they simply can't grasp that someone might be well informed on political issues but have formed a different opinion. Instead, they simply assume that if they can educate them, you're surely come to the "correct" opinion.
#3 DO NOT discuss politics with people who become emotionally involved in the topic. For me, politics is like talking about sports. I certainly care deeply about what's happening in the world. And I have strong opinions about how the problems of the world are best solved. But I don't pretend I'm about to run for emperor. I express my influence on policy through my measely vote and to a slightly greater extent, my blogs that may convince some small group of the value of my opinion.
But some people really get wrapped up in these issues and get very very upset at disagreements. This is particularly true of people whose opinions are based on their feelings. "I just don't like <President X>. I just don't trust him." If they use the phrase "I just..." you're probably dealing with someone who's emotional.
And that's about it. Sometimes I've determined this sort of thing too late -- after damage has been done.
I've had friends who clearly think I'm some sort of horrible, uncaring guy because I don't agree that the solution to poverty is to confiscate the property that one person has earned to hand it over to those who have not earned it. Obviously, I WANT people to starve because I'm an uncaring bastard.
I've had friends who think that I'm an unenlightened ignorant tool because I don't think the invasion of Iraq was motivated by Bush's obvious need to control their oil wells for "his buddies". Why can't I just open my eyese and see the obvious?
As I get older, I tend to have a lot thinner skin about those kinds of things. I don't like having to be the "bigger person" and turn the other cheek and absorb being patronized or looked down as some sort of moral dwarf. So I just try to avoid the topic if it's with someone who has any of the 3 traits I mentioned above.