I have several rules I try to stick by, but before I list them, I'll throw in a little disclaimer:
Different jobs have different interview requirements, and what might be seen as a strength in one role or in the eyes of a particular employer, might be a big negative in other circumstances.
This leads to my first rule:
Do your homework
If you've ever been setup for an interview through a recruitment agency, they always encourage you to check out the website of the company you're interviewing at.
I think that's a given, but my tip is to go that extra bit further. Every other applicant is going to scan the company's homepage too - so dig deeper and look for "conversation points".
Look at things they've been involved in previously, what kind of projects they're currently working on, articles they've been mentioned in... Any resource that gives you more insight into the company.
Not only will it impress the interviewer that you've spent the time really getting to know their brand, but it will also help you on rule number 2:
Sitting back and waiting for the interviewer to initiate conversations is a bad strategy. You'll appear less engaged, and you'll give the employer to much "control" over the conversation.
What you really want to do is lead the conversation into areas you're confident or strong in.
Asking questions also puts you on the front foot. It shows that you're passionate and genuinely interested, and it shows that you can take initiative and have insight that will be of value to the company.
This might not be a rule that applies to every position, but energy and appearing animated in an interview translates into enthusiasm and confidence.
Smile, use positive body language, show that you can be focused and serious - but also don't be afraid to reveal a sense of humour.
My personal trick is to listen to some upbeat music on the way to the interview and also enjoy a can or two of Red Bull before you go in.
If you're competing against several other candidates, your main objective should be to stand out. Try to put yourself in the shoes of the employer: "what is it they're looking for in a potential employee?"
Try to meet that minimum requirement, but also try to sell them on an "added value" that you bring - something they hadn't considered before.
When they look back at how the interviews went and who they should call back, the first name will always be for the most memorable interviewee.