• Be relaxed, be genuine, be yourself.  
• Remember: the EC’s job is to find out more about your personality. Since this is the case, you don’t really need much preparation for questions he/she may ask. Just keep in mind that they’re trying to get a picture of the real you.
• One thing that generally impresses most interviewers is that subject that makes you light up – one that just shows your passion so easily.

• Why do you want to attend MIT? (give reasons, ideally not "canned" ones like "it's a good school" b/c they know they're a good school)
• What are your interests? (keep your answer relevant to math and science unless the interview presses for more info, in which case, elaborate on your extracurriculars or "worthy" pasttimes)
• What do you want to do in life? (relate how MIT is going to help achieve your life/career goals)
• Anything else we should know about you? (elaborate on any main part of your life that wasn't covered; extracurriculars, awards, honors, etc.; or, throw in a funny life story where you learned something; this elaboration should make you stand out as a "unique" person)

Basically, you are looking for presentable, comfortable clothes in which you feel at ease. 

sport coat and gray trousers should do fine. MIT is surprisingly hip so dont look too square, but at the same time dont be too casual. Make sure your cellphone is off (completely). Dress at least in a dressy casual manner (if you e a guy, polo shirt and khaki pants or something similar); if you feel so inclined, you e not going to be faulted for wearing a suit and tie (I did to my laid-back college, and I got in with a full-tuition scholarship). If you have piercings in weird places, now is the time not to insert a ring into them. Cover up any tattoos. Don overdo cologne or perfume (consider not putting any on in fact). Make sure you look clean and presentable. Arrive at least ten minutes early and wander around until about four or five minutes to go and then head to the appropriate person (receptionist/secretary) and politely state your arrival.

• Thoughts on the MIT interview
• Advice on how to approach your interview
• MIT interview tips
• Interview tips from college interviewer