One thing that ought to go without saying is to do your research on the company. Not only does it impress the employer that you've made an effort (and don't be shy about dropping in how much you know about them), but you learn things that will help tailor your responses to what the employer is looking for.
The interweb makes it so easy to find info and press articles etc, but some people still don't do it. I interviewed someone a few weeks ago and they didn't even know what the company does. Interview over.
It's also possible to find out things about the person interviewing you, but you don't want to seem like a stalker.
Internet research works both ways though - more and more interviewers will see what they can find about you. I once changed my mind about a decent candidate after seeing their Friends Reunited entry.