There are certain Neuro-linguistic Programming [NLP] skills that you need to master to be competent in this field. Some of them are:
Calibration: This means reading people's mind. You must have met some people who thought you were upset though you may have been very happy at that time. They failed to properly calibrate your feelings. Calibration involves reading what's going on in a person's mind no matter what physical actions he may be displaying. In the World Poker Championship, there's a lot of calibration. Here, people constantly calibrate people's behavior and judge who is bluffing and who really is in a strong position.
Detection of patterns: Patterns are the base of NLP. But there often lies a confusion regarding the exact implication of the word "pattern". In simple words, pattern means the prediction of events which are to follow, based on what has happened. For instance, a child may observe his dad to eat his breakfast at 8 in the morning. He notices this continuously. The next time, when it strikes 8, he predicts that his dad would come out for breakfast, and he does! That's it, the child has been able to detect a pattern. Then, this skill of pattern detection must be applied for bettering your performance.
Proper questioning: Whatever information that you would get has a direct relation with the kind of question you have put forward. So constantly work on your ability to ask proper questions. Ask questions that are precise, involving, relevant and specific. Besides, there must be full clarity in the question and must not be capable of having diverse meanings. If the question is such, the intended meaning should be communicated to the other party.
Reading the other party's eyes: Search for "eye accessing cues" on Google if you do not have a fair deal of idea regarding eye accessing cues. As far as my knowledge is concerned, this term eye accessing cues is used exclusively in NLP. As you become a master of this skill, you will be able to relate to the other person, and feel what he might be thinking. You also will be able to appreciate what they might be thinking about at the back of their mind.
Having mentioned a few points essential for developing your NLP skills, it must be said that these are not exhaustive. The four points mentioned above just list the basic requirements where you should focus your attention on, by regular practice. There are several other points which you must adhere to!