i had mine last week.
1. just be yourself, and relax. they can smell bs a mile away. when i say relax, i don't mean apathetic. get really excited about things! they love that.
2. i wore a suit. normally that's a little too much, but my interview was in my EC's office, where everyone else was dressed up. dress appropriately for your interview location. if its starbucks, dont wear a suit.
3. dont JUST answer the questions. make sure you answer the questions first, but go off on tangents! and then ask the interviewer questions too!
4. dont talk about grades, test scores, etc. that's what the rest of the app is for. I MEAN IT. talk about other stuff.
5. dont research your EC's job. the less you know, the more questions you will have, and questions show interest.
6. if you've ever been to mit, talk about it. talk about things that are unique to mit. i did the mites program last summer, and i talked about that.
7. show that you want to go to mit more than anywhere else. idk how, just figure out a way to do it. my EC told me that she felt like her other interviewees were only applying because their parents made them. DONT be one of those. show passion.
8. bring a resume or a guide, NOT for your EC but for yourself. you might lock up and forget all the things you do outside of school. feel free to offer your EC a copy but he/she will likely not accept it.
9. bring STUFF. and by stuff, i mean bring pictures of work that you've done, things you've made. if you program or do web development, you might even want to bring a laptop. if your an artist, bring in evidence. i've heard submitting supplimentary material can hurt your application. the interview is a free time to show mit who you are. you might not even take any of it out, but bring it anyway.
10. prepare some questions for your EC about mit, but dont read them off a paper. memorize them. bring a notepad and take notes.
11. don't try to get it over with. if anything, try to make it last at least an hour. at least. they need to learn enough about you so they can write up a good review.
12. dont apply to mit and make me look bad with your awesome grades and extracurriculars. jk jk jk
13. see #1, #7, and #4. those are the biggies.