Be confident. You applied to this job because you think you’re the right person for the job, and they’re interviewing you because they think so too. This is more of a formality to confirm both of your suspicions.
Make eye contact (that’s what confident people do).
Smile.
Be friendly and aware (e.g. laugh at their jokes).
Be excited when talking about what you've done. You’re proud. If you’re not, the interviewer has no reason to think your accomplishment was impressive.
Come prepared with interesting stories about what you've done and past job situations. It’ll help keep you excited, as well as confident because you won’t have to fish for stories and interesting tidbits on the spot.
Study the job description well before interview. Bring a copy.
Based on the job description, come prepared with stories that connect what you've done with what they do. This is crucial because a major goal of the interview is for them to picture you working with/for them.
They will ask you if you have questions for them; do not waste that opportunity. You want to show you're analytical and thinking of the big picture of their business. This in turn shows you’re the type to take ownership - allowing them to picture you working with/for them.
Consider making at least one of your questions related to current events about the computer, whether a press release, news directly involving the company, news related to the industry, etc.
Consider making at least one of your questions related to industry trends, how the company’s direction aligns with your thoughts on the trends, and ideas you have for the company to better align with/capitalize on industry trends.
If you’re also far along in the interview process somewhere else, but think the job you’re currently interviewing for is a better fit, tell them so if they ask. It will be to your advantage for them to realize that you’re wanted by others but you would choose their company if given the opportunity. I believe that letting them know that would generally make them consider you as more of a priority candidate, if they were already interested in moving forward with you.
Mail the interviewer(s) a thank you note that day. It’ll get to them over the next 2 days or so while you’re still fresh in their minds, and often before they made a final decision. You will stand out, and I believe very positively. It shows them you take initiative, and helps them to picture you working there, doing what it takes to get your work done in the best way.
Follow up with the primary interviewer if you didn't get the job to see what could've been done better. But make it clear you're already moving forward with another place and just want advice to improve. This way, they'll be less afraid of lawsuits and more likely to give an honest critique.
Don’t be late. You know this is a heavily scrutinized encounter, and they know you know that, so any easily avoidable missteps like that are damning. Better yet, come early. You’re responsible and dependable.
Dress well. If you don't have a shirt that looks sharp and you feel confident in, go shopping for a shirt/tie or 2 with someone who does. Buy many for you to wear while on the job. It’s good to look sharp and well-dressed.
I don’t think I’ve ever interviewed without a tie, even in laid-back tech companies. Not everyone would agree, but I think it shows seriousness. Obviously don’t overdress. I think the common rule of thumb for an employee - dressing in the top 10% of employees - would apply to interviewing as well.
Consider hiring a resume review service. If you get the right job partially due to such efforts, the money will be well spent and dwarfed by the value you’ve added to your life. They also run Groupons for such services regularly.
Consider using an interview critique service for the same reasons. Practice can only improve one of your main goals - being confident. They also run Groupons for such services regularly.