Strike a pose
During a portrait shoot, it is your job as the photographer to put your model at ease. Chatting to them before you start will break the ice and instantly put your model at ease. Just remember to keep it going throughout the shoot, long silences can leave the model uncomfortable and feeling ignored.
Let your model decide
First, for tips on getting your model to feel at ease, read this article; Briefing a model. Once they are feeling comfortable with posing for you, try to let the model lead on determining their pose and position. Too many instructions can be confusing, let them settle into it naturally and make small tweaks as you go on.
Look at this sequence of shots. You can see how the model tries different positions of head, arms and expression until the photographer is happy with the shot. It is very difficult to instruct a model into the perfect position; the easiest way to achieve good position is to ask your model to try many different positions, shooting as you go.
Let's take a look at another portrait sequence:
In this set of images the photographer has the model sitting down which increases the options for hand and arm placements. It is often easier for a model to find a comfortable position when they are sitting or leaning rather than simply standing. Giving your model something to do with their hands helps with posing.
One final one to look at:
In this final sequence the photographer simply asked the model to walk towards the camera and enjoy the natural surroundings. Having the model walk put her at ease and allowed the photographer to get a very natural set of images without any formal posing at all.
A portrait lens helps you to shoot from close in, see the range in the Canon Store.