I had the same problem, I'd been job hunting since April this year and had quite a few interviews but always went to pieces and clammed up.
I was working for an Agency on a temporary assignment (no interview!) and the director heard that I had another interview coming up and that I was rubbish at them and she offered to help me.
She had a look at the job description and then went through with me answers to the questions that she thought they would probably ask, based on the job decription and the person profile. She give me hints as to what to say, how to phrase things, what kind of examples they would be looking for, advised me to use the same words that they had put in the description etc. She then wrote it all out on a flip chart and told me to take it home and study it the night before.
Next day I had the interview, was far more confident than in previous ones and it obviously worked 'cos I got the job!