Based on several recent conversations with friends:
Rule 1: Don't assume you know the background of the person you're talking to. Judging someone by their appearance isn't fair, nor liberal. Disabilities don't always show. Culture, religion, background rarely show, nor does gender orientation. Pigeonholing people is not very tolerant, nor liberal.
Rule 2: Anyone can discuss an issue. They may not be correct, or they may not agree with you, or some of each or both. Telling people they don't get to discuss an issue is not going to convince them you're right. It's only going to convince them you're conceited.
Rule 3: If it's a technical subject, such as firearms, reproductive biology or even a religious faith, the technical knowledge is a necessary part of the debate. Don't claim to be "informed" if you can't answer basic technical questions on the subject. In such cases, it would behoove you to acquire some technical knowledge, or to consult with someone who does. Here's the tough part: You have to assume they know what they're talking about, and believe their statements, much as you would a professional in any field. If the facts make your position uncomfortable, then perhaps your position needs to change. You are entitled to your own informed opinion. You are not entitled to your own facts. And if your opinion is not informed, you're still entitled to it, but no one is obligated to take you seriously.
3A: If you don't trust a person's technical opinion, why did you ask for it? Getting a second opinion is fine. Asking other people who are ignorant of a subject to fact check the expert is not only insulting, it indicates you need to re-read this rule. If most of the subject matter experts tell you you're wrong, maybe you are. This doesn't mean you have to like the conclusion. You only have to accept it.
Rule 4: Once you've demonstrated bias through ignorance, your credibility on any other matter drops fast. See Rules 1 and 3.
Rule 5: It's entirely possible for someone to have the same or different sources, be informed, and come to a different conclusion. This does not mean either you or they are wrong. Few issues are binary in nature. This offers an opportunity to debate, share information, and improve your position, even if you never fully agree with the other party. And that's okay, because liberals are tolerant of dissent.
5A: Don't assume that someone disagreeing with you must be ignorant of the subject. Ask questions. Learn why they have their position.
Rule 6: If you are absolutely sure the other party is wrong by your interpretation, remember that you also don't know everything, and even matters of common knowledge change with time. This person may be comfortable in a previous or foreign culture, or they may be ahead of the curve and ready for a future society. More likely, the answer lies somewhere between.
Rule 7: Activists don't have to compromise. They're expected to be extreme. If this applies to you, remember it also applies to others. They're entitled to the same focus and drive you are.
Rule 8: Skin color only matters to people who care about skin color. A person with dark skin may be from somewhere Africa, the Caribbean, or the US or anywhere else. Each of these people will have a different background. Their skin color only affects how you treat them, and how you treat them affects how they respond as a person. If you make assumptions about them based on their skin color, you're being presumptuous. Likewise, a white skinned person may be from the US, from Europe, from Australia. They're not going to all be the same, either. In fact, even American geography matters. However, it's likely that two Americans (or Canadians, Aussies, Greeks, Chinese), even of differing appearance, will have more in common with each other than with two people who look the same from different nations. If this is a revelation to you, you may not be as unbiased as you like to think you are.
8A: If you're a middle class white American, lecturing people on how middle class white Americans shouldn't lecture people on race/color/culture, you may have run into a recursive logic failure. Also, if you're assuming from visual cues that the person you're debating with is a middle class white American just like you...see Rule 1.
Rule 9: Telling someone else how they think or feel is conceited, derogatory, and wrong. Please don't do it.
Rule 10: You know how the popular press always gets your subject wrong when they discuss it? You know how the opposing press leaves out key facts through error or deceit and presents you in a bad light? It's a good idea to assume the same thing happens with other subjects and to other groups. See Rule 3.
Rule 11: Epithets like "right winger" and "wingnut" and even "troll" don't encourage the other parties to continue discussion. They're also neither liberal nor tolerant. Unless you plan on exterminating or outbreeding the dissenters, you need to patiently try to persuade them. Some won't be persuaded. This is not your fault. It may not be their fault, either. All you can do is try.
Rule 12: Selection bias doesn't help. Seek out opposing viewpoints. You may change your own. You may strengthen yours. You may change or strengthen theirs. Worst case, you'll know how the opposition thinks and feels, and so will your allies. Holding carefully monitored discussions with your friends isn't a bad thing and has its place. But it's not an open discussion. Keep private matters private, and allow public discourse to be open. And echo chamber is not very useful.
Rule 13: If you're thinking of deleting, blocking, shutting down a dissenter or a thread because you don't like how the discussion is turning, you may not be as tolerant as you like to think you are. Certainly there are absolute trolls who contribute nothing. But a dissenter is useful under Rule 12.
Rule 14: Shocking fact: These same rules apply to conservatives. You may in fact find fora where you're tolerated and treated with respect. You also may find some where you're insulted and called a troll. You can complain about this...if you haven't committed the same act yourself.
Rule 15: Nothing is binary. There are pro-gun gays and feminists. There are anti-abortion atheists. There are black racial separatists. There are pro-gay Muslims and Pentecostal Christians. There are polyamorous Republicans. See Rule 1.
Rule 16: Even among people of the same general background, individuals have different experiences and perspectives. What's wrong for you may be right for them. Or it may simply be they're a product of their experiences.
Rule 17: If everyone agrees with you, you need to widen your circle. If you're not angry at something at least once a day, you're probably not learning anything.
Rule 18: Yes, it gets frustrating re-hashing the same material. If you've adopted the mantle, you have to try to be patient. Unfortunately, it's the only way. See Rules 11 and 13.
Rule 19: Some of your opponents will love you even though you're wrong. Try to extend them the same courtesy.
Rule 20: There is nothing morally wrong with apologizing. If you were perceived as offensive, or lost your temper, or even just misunderstood, apologize and try again. Often, that will make more progress than any number of graphs and charts.