Hi,
I did my fair share of studio photography with models and yes it is not easy. The utter most important thing is to get your model to relax. Here are some things that I found very helpful:
1. Introduce yourself. Very important! I have seen it so many times that photographers just get right into action ... Take 5 or ten minutes to talk to your model. Try to get a basic idea what kind of personality she is and be interested in what kind of shots they like. You will be mean and reuse this information in 4.
2. Have a clear vision what you want. Talk to your model what kind of image you want to shoot. Talk to her about the feeling of the image (moody, aggressive, dreamy, ...) If you have an inspirational image, show it to her so she gets an idea of what you are looking for.
3. Get the model in position. This is sometimes a little difficult, but don't touch your model. This is especially important if shooting girls or kids. If you know the model and have worked with the noel a couple of times it can be ok, but never just reach and touch. Always tell them what you want to do and ask for permission.
To hep them you can either show them the pose you are looking for yourself, or you can direct them by giving clear directions (e.g. look at that point on the wall, look at a spot 5 inches higher...)
4. Once you have the position you want. Get your model in the right mood. A little trick is make them remember certain situations or things. If you want them to smile you could tell them 'smile' or you could tell them 'think of your most favorite food'. You can tell them to look 'flirty' or you can tell them to 'imagine you are sitting in a caffe shop and then this cute boy/girl comes in and you are trying to make eye contact' ...
Stuff like this. When done well you will see that a smile will not stop at the lips, but involves the eyes, etc ...
So use your 5 min chat with the model in the beginning and try to adjust the stories to her/his personality.
6. Be confident! If you are not confident, your model will not relax. If you realize after a few frames you had the iso set to a wrong value, don't make a fuss about it. The model must feel your confidence in order to let go. Especiallyif shooting digitally, don't look at the screen of your camera every 2 shots and say 'hmm...' or 'stupid...', 'not quite...'
7. Keep the connection! If you so it right you will see that your model starts to relax at some point. They start to play with your camera and you feel a magic connection. Don't break that. Keep shooting! Have either a person at the shoot to reload your cameras or if you have a camera with multiple backs, have them lined up. If you are in a position to use multiple camera bodies so it to keep interruptions at a minimum.
I guess the list goes on and on and on ... But start slow, build up your arsenal of tools. Working with models is really something you have to learn. Learn some basics for posing ('If it bends, bend it"...)
Learn about how certain poses are better for men and women, how certain poses are better for certain body structures, etc.
Most importantly? Have fun and let your model see and feel that you are having fun!