dont fold your arms and only undo your suit buttons if they have theirs undone too. Good luck.
Agree with first part, second part is rubbish.
If it's a single-breasted suit the jacket buttons should not be done up, unless you're some sort of a complete plonker.
Have a list of 4-5 questions you want to ask them about the job prepared for the end of the interview. This is as much you interviewing them as it is them interviewing you. They will be pleasantly surprised if you pull some good questions out (even 1-2).
Make them open questions - how why what ones.
Ensure you have the names of each of the interview panel and look them up on LinkedIn. Pre-stalk them on Google, too and see if their profiles give you any clues to their interests and play to the cloud accordingly. You also want to know who's the most senior before you go in and figure out the influences between the three while you're in there.
Ask if there's anything they would like you to prepare for the interview beforehand.
Don't be afraid to say 'I don't know the answer to that, but I'm happy to get back to you, if you'd like'. Sometimes it's not knowing all the answers that matters, it's showing that you are willing to admit you don't know, but will sort that knowledge out.