Most importantly, be yourself. If you walk into an interview consciously trying to be something you’re not, there will be a lot more pressure, which will make you more nervous. But obviously remain professional too, especially with mannerisms as others have said. It definitely helps if you’re interviewing with people that are a little laid back, but that can’t be helped. Just relax, smile, and do your best!
I also recommend doing research on the company and perhaps the person or people you would be interviewing with. It shows that you put in some effort and actually care about the company you might potentially be working for.
I always bring copies of my resume (and cover letter if applicable) as well. It shows preparedness if they weren’t able to print a copy for themselves to reference. Have any questions you might have for them prepared in advance too.