Before the interview, read up about the company - find some specific reasons you want to work for *that* company, read some news or new developments and make sure you mention them when they ask you 'what do you know about our company?'. Dress very smart, and don't cover yourself with papers, bags, folders, hair.
During the interview, make sure you answer their questions directly, elaborating a little. Don't talk too much, but give enough detail to back up your claims. Keep good eye contact, and if there is many interviewers try to divide your attention between them.
After the interview, they will ask you if you have any questions for them. Prepare some beforehand, and don't be afriad to read them off a notebook, and write down their answers. Make sure they're good questions that are suitable for the role! If you're in sales, ask about targets, what the top performer earns. If its a junior position, ask about progression.
Finally, if you can, try to ask for a business card(s) of the interviewer(s). When you get home, write them an email thanking them for their time and confirming your interest. Make sure its well spelt, grammar checked ... etc ;)