Don't lose concentration at any point. Don't start analysing your answer to a previous question whilst listening to the next. Most of all don't feel discouraged and give up on the interview if, for some reason, you feel you aren't performing as well as you could. The best way to talk about your strengths is by looking carefully at what the employer is looking for in an employee, and what skills, competencies or personality traits you have to offer. Do try to follow your statement by a short example demonstrating that particular strength in action.
Weaknesses are slightly more tricky to handle.
Think of a weakness which is not going to put serious doubt on you being the right person for the job, and whatever weakness you choose to disclose make sure you stress the lesson you have learned and how you have corrected, or intend to correct, this weakness. Nick Lane Interviews are almost always scheduled during working hours and even though some employers are fairly flexible in offering you more than one time slot, the choice is usually within working hours. If you are facing several interviews at about the same time why not consider taking a leave of absence for the duration. A more relaxed timetable will probably improve your performance and will help you get the job you are aiming for. I always get asked this and I always struggle to find a good answer.