Also, humour them a little bit. Once, my Dad was in an interview, he told me they asked him what his weaknesses were, he replied with 'Mars bars'. I think he got the job, however I'm not too sure.
That's fairly terrible advice!

If they inject humour then you can go with it but you aren't chatting up a girl here.
Loads of questions online so look for ones related to what you will be doing and tailor your answers. If it's customer facing then try and highlight your people and problem resolution skills-or at least have an answer to a 'give an example of how you dealt with a difficult situation in past'.
If they ask 'describe yourself' again tailor it to highlight what makes you suitable for the job and be able to back it up. Also if you submitted a CV be sure you can back up everything in it as a lot of questions could be based on what you put in there.
On a more basic level. Ask for water if it's not offered, it will be, and if you need to wear a suit, probably not needed, then take off the jacket or you will likely end up pumping sweat and feeling uncomfortable and making the people interviewing you uncomfortable.
Good luck with it.