There ought to be at least two people on the interview panel IMO, especially in a professional setting.
Only27 I'm getting the impression you're getting lots of interviews (well done on being short-listed) but have not yet got a full-time offer of work. Don't let this get you down, it happens. Keep on working at it. Yes, shake hands, I always did, unless there are so many on the interview panel that it would be ridiculous, and smile. Don't sit grinning like a Cheshire Cat of course.
We expect candidates to be nervous, in fact an over-confident candidate can be a put-off.
Give yourself time to consider the reply to a question, don't just jump in and hope the answer will come to you. Keep your answers succinct and to the point. Don't waffle on, but give enough information to show you know your stuff and include one or two concrete examples.
Address your answer chiefly to the person who asked the question, but also look,at the other interviewers and watch their reactions. You might see one of them smile or nod if you make a good point. If so, give that persons a small smile of acknowledgement.
You are, I think, looking for your first FT SW job? So emphasise your enthusiasm (don't over do it) and your willingness to learn, and to undertake any further training they wish you to.
There's always that final question such as 'is there anything you wish to ask us about this position' that's a good time to ask if there will any further on-going training opportunities. Believe me, professional organisations like their staff to be up to date and willing to go on courses.
If you're keen enough, and keep on applying, you will get a FT position in the end. But in the meantime use any temporary posts to gain experience and make contacts.