Your interviewer won't give a shit what color the tie is. If it stands out too much, it will count against you. They will discuss each of you after you leave, which means your name being tossed around will be based on your NATURAL ability to relate to the guy asking you questions, appearing at ease during the questioning, and your ability to convey a true interest in helping others.
Body stance should include NEVER....NEVER....NEVER crossing your arms. Keep your hands on your kness or in an open position. It conveys being open. Crossing your arms conveys either your attempt to hide something or an attempt to keep others out of your personal space. In other words, you will look shady.
Do NOT fake your personality. If you don't find a joke funny, don't sit there laughing for 10 minutes just to kiss the interviewer's ass. They see tons of guys like you and I am sure they get tired of it.
Realize your interviewer is human. I had one who became my true friend later on. In fact, his home phone number is still in my cell phone and I could call him tomorrow and he would ask why I haven't visited. His personal character traits include telling the same jokes for the past better part of a decade that I have known him. I still smile at them, half "chuckle" and move on.
Accept the personality of your interviewer but don't kiss his ass. You are often much more transparent than you may think, especially to an academic.
Other than that, relax and play yourself up. Mention community service, extra curricular activities and such. They already have your grades. They don't really need you to act as if your GPA is all you have to offer. There are millions of people just like you.