Let go of trying to get the other person to do what you want. It's easy to get into this mode, especially when you're feeling frustrated. Trouble is, it usually doesn't work, and can make things worse when the other party decides to withhold trust, and dig in their heels as a result. Even if you succeed, they are likely to give you only what they have to, and will get back at you at the first opportunity. Sometimes you may think you have "let go" of trying to control when you are still sending judgmental messages. You might say, "I felt manipulated when you slammed the door." Your "I" statement here may have an assumption imbedded in it about the other person. You may believe that they are angry with you, or trying to intimidate you. This type of "I" statement is more likely to elicit a defensive response as the other person tries to ward off the assumptions they believe you implied. They are simply trying to prevent you from controlling them. What you can do is control how you react to them. You can accomplish this by asking genuinely curious questions. Sharon suggests using the "who, what, when, where, why and how" questions to gather information. Using the above example, you might ask "When you slammed the door, were you angry at me?" When the other person can see you are really interested in how things look from their perspective, it gives them permission to relax as well.
Treat your own and the other person's ideas about each other as subjective beliefs, not immutable truths. No one knows everything, and everyone knows something. If you act on this knowledge you open the door to getting more information, which will help you make better decisions. This doesn't mean you should assume you're wrong or don't know anything, just that you're open to new information. If people sense this about you they'll be more likely to try to work things out with you than to fight to the finish. Also, as Sharon says, most conflict is based on people's different experience of what's happening, not on differences of fact about the situation. Asking questions that surface the other persons' experience helps prevent defensive escalation. One way to accomplish this is to ask the other person what they believe about the subject of your dispute, or what some of the things they've said or done mean to them, or about their motive for what they've said or done. In the above example you might ask "What caused you to slam the door?" Of course, you have to actually be interested in the information.
Put your assumptions on the table. Often, we embed our own assumptions about the other person in what we say without being explicit about them. This leads them to react even more defensively, because they sense our hidden agenda even when they can't articulate it. Often, they will attempt to resist our hidden assumptions so they won't be backed in a corner and "lose" somehow. It's better to put your assumptions openly on the table. One way to do this is by stating your assumption and then asking them whether they agree. Again, be curious. "I assume you're angry, aren't you?", sounds accusatory, and may lead to them denying it even when it's true. "When you slammed the door I assumed you were angry at me. Were you?", leaves it more open and more likely that they will tell you what you need to know. Notice that this way of saying it has three parts. "When you slammed the door…" (how you got to your assumption); "…I assumed you were angry with me." (your openly stated assumption or conclusion based on what you saw); "Were you?" (your request for clarification from their viewpoint).
Be genuinely curious. If the other person senses that you really are gathering information in order to learn, rather than to stick it to them, they will be much more likely to respond in kind. You can accomplish this by being aware of your body language and tone of voice, and by doing whatever helps you to calm down in the moment. Doing this while following the first three steps above helps because it demonstrates in the moment that you are open to the other person, and not just trying to defeat them. The choice for them to react in kind is now easier.
Another benefit is that you can hold the other person accountable for their behavior as well. Saying what you saw in the above example puts their behavior on the table as well as your assumption about it. This can be helpful in a case where the other person is denying having any feelings about a conflict. When you state the action and your assumption non defensively, they can give you better information: either informing you of how the action wasn't the emotion laden event you believed ("I was distracted and the door got away from me"), or confirm and expand upon or correct your assumption ("I was more frustrated than angry") without feeling like they will be verbally punished for being honest.
Dealing with difficult people can be unpleasant; it is also easier than you think. Being willing to learn, and asking the right kinds of questions helps, as does being aware of your demeanor and intent. More honest and open communication on your part makes it more likely the other party will react non-defensively.