Rescue Remedy! Probably a load of cobblers, but if you believe, I'm sure it helps.
Also, take your time to answer questions. Gather your thoughts first before trying to blurt something out.
Good eye contact, not too much though. As a guideline, keep eye contact with someone while they are talking to you, then make eye contact with everyone whilst you are answering.
If you've a job already, then just think you've got a fall back if things go wrong. This stopped me getting too nervous.
Most importantly, just be yourself.
Unfortunately, nothing is a guaruntee of getting the job as every interview is different. Interviewers all use different techniques to decide who the person is for the job.