I have a whole litany of advice that I could dispense based on particular interviewing situations that people find themselves in, but there are two things that I always remind myself before going into any interview:
(1) Be as likeable as possible
(2) Have the employer talk aloud about the role they’re trying to fill and the problem they’re trying to solve, and make them know that you are the exact solution to their problem and they will solve that problem by hiring you now.
The most useful advice I got starting out was to practise responding to likely questions out loud before the interview. Make a list of say 50-100 likely questions you will be asked under various sections such as education, work experience, personality traits etc (there are lots of books available listing common interview questions) and then record yourself on your computer answering them out loud.
Then play it back. The first time you watch it, you will probably squirm a bit with embarassment but then repeat the process and try to improve. Pace your responses. Don’t stumble over your words. Don’t fidgit (people often subconsciously twist their ring/watch, look down etc - be aware of what your body is doing).
Don’t start a reply with emm, eh, etc. Take a second and begin your response smoothly. If asked a question you haven’t prepared, relax. This is when people can really freeze. Take a second and say, ‘that’s a really good question, I hadn’t ever considered that before’ or something similar. Then do not BS the person. If you don’t know, it’s ok to say so.
If you are just starting out career wise, lack of experience is almost always the best weakness to admit to. That is definitely one question you do not want to be too honest about!
Numi’s tips above are solid. To expand a little, I think the main point is to try to sell how you can be of benefit to the company. So it’s no use saying how much you’d like the job or what a great fit it would be for you. You need to tell them why you would be a great fit for their company.
Also, try not to make vague and general statements such as I am a hard worker, or I am a good people person. Always have examples on hand that you can explain a situation where through your hard work or people mananegement skills you were able to produce a good outcome for your current/former employer. If you are a student, you can use examples from summer jobs, internships or college group projects.