First of all, before you even apply, make sure you've read the job advertisement and description carefully. In particular, you'll want to make sure you meet any requirements mentioned in the advertisement. If there are areas that you do not fully meet, be sure that you have positive answers for those.

It always creates a good impression in an interview if you know something about the company and the market they operate in. Do a little research about the company products (or services if it is a service oriented company), staff, clients, and competitors. Also make sure you know the names and job titles of those who will be interviewing you. Some companies have one-to-one interviews, but others my use an interview panel.

Punctuality
Punctuality is crucial. Turning up late creates a terrible impression, so you must be sure you can arrive by the appointed interview time. Plan your journey in advance. If you are travelling by car, make sure you allow for traffic delays and that you know the route. Look at a map of your intended journey. If you are travelling by public transport, remember that services can be subject to delays and cancellations. Always make allowances for these.

Dress for success
The way you dress says a lot about you to a prospective employer. You will have a head start if you create a good impression with your appearance. Remember you only have once chance to create a first impression. You should dress conservatively, especially if you will be working in a corporate office, but check out the office dress code too. If in doubt, it's better to dress too formally, than too casually.

Make sure you check that you are looking well groomed before going into your interview.

Your Resume/CV
Have your resume/CV, references and any additional relevant information with you. Even if you have already mailed these with your application, you may want to refer to them during your interview. Not to mention that it makes you look seriously unprepared if you don't have them with you!

Read over your resume/CV and make sure you know it inside out.

The Interview
Give a firm handshake when entering. Enthusiasm is infectious, so be enthusiastic about your achievements when asked interview questions. Try not to talk too quickly, especially if you are a little nervous, and do not interrupt your interviewer when they are speaking.

Try to put yourself in the place of the interviewer and imagine the kind of questions they would ask, and prepare model answers for any awkward questions that you believe they may ask you.

As the interview draws to a close, it is very common to be asked if you have any questions you would like to ask them. Prepare your own questions in advance, which might include - What is the department business plan for the next 1 or 5 years? What challenges do you foresee and how do you plan to overcome these? What challenges do you envisage in this role?

Ask about the follow up procedure if you don't already know. Find out when you should expect to hear, and whether they intend to conduct 2nd or 3rd interviews etc.

Follow up letter
It always creates a good impression to follow up the interview with a letter thanking the interviewer for their time and reiterating your suitability for the post.

Unsuccessful
If you are unsuccessful in your job application, it is worth a call to request feedback on how they thought you performed in your interview. Try to turn your disappointment into a drive to use this as an opportunity to improve next time.

Successful
Well done! You got a job offer! In your rush of excitement, however, don't forget that there are still things you need to keep track of. Before rushing out to quit your current job, you should wait until you have received your job offer officially by post. Make sure you have written confirmation of your main terms of employment, including salary, hours of work, main responsibilities, etc.