Smart clothes, not necessarily a suit, but definitely smart business like attire.
I second using "I" instead of "we".
Be very familiar with your CV, be prepared for any questions that may come off the back of it.
Do some research into the company. Read the job description and make mental notes of how your skills and experience make you a good fit.
I've recently had a few interviews and also conducted interviews at work, all now using competency based questions. A few reoccurring questions:
Talk about a challenge you've faced and how did you tackle it?
Talk about a time you've helped a colleague
Talk about a time you've made a mistake at work, how did you find it and what did you do to resolve
Talk about times you've dealt with conflict at work
Go into the interview feeling confident. Just remember you're there because they fee you have potential to be a good fit. Also remember, you're interviewing them as much as they are interviewing you. It works both ways! Good luck!