It's OK to pause and think before you answer a question (although obviously, don't be silent for minutes on end!).
It's also OK to ask the interviewer to rephrase or explain if you don't really understand the question.
Try to give examples. So if they are asking about organisational skills, for instance, give a practical example of how you went about organising something. And it's absolutely fine to use examples from your 'real life', not just your work experience.
When you are preparing for the interview, write down a list of questions. Come up with questions that show you have researched the organisation and industry. It's always good to ask about training and development opportunities and the company's future plans - it suggests that you are looking to make a long-term committment. If you discuss your questions during the interview, tell the interviewers this when they ask if you have any questions - you can say, 'Well I was going to ask about X, Y and Z, but we've already covered that'. Or you can ask them to expand on something that you've already discussed - 'I was really interested to hear your company does X. Can you tell me more about that please?'. When you ask your questions listen to the answers and respond - try to turn this part of the interview into a two-way conversation. Acknowledge what they are saying, use it as an opportunity to ask more questions.
Always take a notepad and pen into interviews. It is perfectly acceptable for you to make notes. So if an interviewer does say something that you want to ask about later and you think you may forget, or if a question suddenly pops into your head, write it down.
Try to remember that interviewers are often as nervous as the interviewers. It's true, honest!
If you get an interview but not the job, always ask for feedback. You may find that you get really constructive information about things you can improve next time. Or you may learn something that boosts your confidence immensely. I was once told that I was second choice and that they really struggled to make the decision. On one hand, to only just have missed out was quite disappointing, but the positive stuff also gave me the confidence to reapply for a similar job at the same company which I then got. I probably would not have bothered if I had not asked for feedback.