Nothing offends the officers I know more than people dressing like they are going to a flea market rather than an important government office. The ISOs believe they are important, and they are, and the way you dress shows them what you think of them. If you dress poorly, the ISO may take offense, and that is a bad way to start an interview.
Dress as if you are going to church or to a good golf course. No shorts, tank tops, mini skirts or after six wear. Please wear khakis, button up shirts, and shirts without writing on them. Do NOT wear a t-shirt or cap extolling the virtues of your country.
Prepare for the interview
You will get a letter from CIS telling you exactly what to bring. Go over the list and collect copies of everything you need, while keeping the originals so that you may show them to the ISO. If you will be facing questions about your relationship, make sure you and your spouse quiz each other about details of your life to simulate the interview.
Be early, but not too early
15 minutes ahead of your interview time is fine. Any earlier is overkill.
Even if you are in a real marriage with 5 kids, your interview will still be tough if the ISO notices that you and your wife arrive in separate cars -- unless you have a valid explanation.
Be nice and smile.
This is your best chance to control the officer's mood - -show you are nice and the ISO could be nice back. If you are scared, or angry, or uncomfortable, the ISO will wonder why, and we don't want them to wonder.