Good portraits are usually the result of collaboration between the photographer and the subject. It's vital to establish a rapport. So here are a few tricks of the trade which can come in handy - in non-photographic situations as well.
Firstly, when you shoot a portrait you should have everything prepared. Nothing breaks down a relationship faster between subject and photographer than fiddling around with lighting and camera settings - unless you can do it while keeping up a healthy banter. Actually the fatal error here is ignoring your sitter.
When you photograph somebody they should feel like they're the most important person in the world. Being in front of the lens doesn't come naturally to most people. Lots of experienced models I know still feel vulnerable in front of the lens, until you put them at ease.
So there we already have two important principles. Don't ignore your sitter and do everything you can to put them at ease.
How do you build rapport quickly? Well I'm not a psychologist so what I say here is based on experience rather than scientific theory. If I can then I try to find out something about the person I'm going to photograph, what their interests are, what they're passionate about. Then I think of a few intelligent questions to ask them. It helps to have a sense of humour and a tasteful joke up your sleeve too.
Once you get someone talking about their interests, listen carefully. I am a sponge for information and take an interest in most things. Being a good photographer is knowing a lot more than just about cameras, lenses and lighting.
Now here's a golden tip. Pick up on the last phrase your sitter says, repeat it and turn it into a question or a statement. This will help you lead the conversation on smoothly. For example the sitter explains how they love sailing their yacht and ends the description by saying, "... and we sailed into the harbour under full spinnaker." You would then say, "Under full spinnaker? I didn't think they allowed yachts to come in under spinnaker." Then pause and your sitter will naturally feel inclined to explain their point ... keeping the conversation moving.
Just remember. It's all about them, not about you. If they want to know something then they'll ask. The trick is then to give a concise, appropriate answer and steer the conversation back to them. For heavens sake don't go on about all your achievements, or even worse about all the problems you may have. The last thing your subject wants to hear about are your problems and difficulties.
So there you go. A non-photographic photography blog. But I hope you found it useful anyway. As I always say there's a lot more to photography and getting the shot than just the photographic part.