Do your research so you know as much as possible about the company. You should be aware of their work and their clients, be sure to keep abreast of any news about them in the press, including new wins or awards. Do you know what the company is planning in terms of future growth strategy, or different areas of work? You should also get a feel for their culture and what they seek in hires, although you should draw the line at Facebook stalking.
Look your best. Dress more smartly than your counter-parts (they’ve already got the job!). By wearing something comfortable that you know you look good in, you will feel good too. Probably not the best time to break out the new shoes that you can barely walk in…
Find out where it is and work out your route in advance. Don’t aim to be on time, aim to be early. That way you allow for any eventualities and it'll give you time to sample the local coffee and get yourself together. It doesn't give the best impression if you show up 10 minutes late, flustered and saying someone pulled the alarm on the tube.
Make sure you eat something. I know it sounds obvious but you don’t want your belly to be grumbling while you are trying to concentrate on the interview.
Take a hard copy of your CV just in case, and keep a note of the name of the person you are supposed to be meeting – it could be embarrassing if your mind goes blank and you forget the name of your interviewer!