You probably already know this and i hate the thought of being patronising but this is my best advice i could give anyone!!
Always smile
End every answer on a positive note
Ask questions at the end when they ask you if you have any questions. It shows that your interested in the post and they like intitiative. Something like, what benefits are available to me such as pension plans holidays etc, or ask why the postition is available and why was it created.
Dont be afraid to discuss your weeknesses and pet hates when asked just end up saying how you spotted your problems and how you resolved the issue in the workplace.
Most important BE YOURSELF!!!
Ive just read an article from a professional engineering magazine in uni about impressing at interviews. These were the main pointers.