So, you got an interview for Y Combinator ("YC").
Congrats. It's always very competitive. I don't think the partners would like me revealing current selectivity, but the interview filter was about 10% back in 2008, and it's even more competitive now.
It's incredibly fast. You will deploy a very small fraction of what you prepare for, but you still need to prepare a lot.
One of their big turnoffs is if they think they can identify a problem or angle you haven't thought of, so have an answer for everything (avoid "I dunno - good point"). Therefore, in this regard, it's in YC's best interest for you to follow Sam's advice of, "If you don't have an answer, be honest", but not really your best interest. As a side benefit, thinking on your feet will reveal intelligence. You still have to be open to their proposed changes. PG's spoken before about the balance between confidence and being open to ideas in the interview.
Put yourself in their shoes: They are conducting so many interviews in a day. It's very repetitive and mentally tiring. They struggle to remember you, although they take pictures and video to try to combat this. Do something weird/clever/creative, give them something (snacks?), or wear something memorable. I believe the AirBNB founders brought their ObamaO's, demonstrating resourcefulness in a memorable way. PG notes "The best way to stick out is to seem like you deeply understand your domain and your users" but I assume every team is trying to do that anyway.
If you're an MBA, avoid MBA-ish answers and just use straightforward language. Have the hackers on the team speak as much as non-hackers. Know the typical boring answers for chicken-and-egg problems and customer acquisition, but definitely have an additional crazy idea prepared for both.
Show a LOT of determination. Back it up with evidence of commitment, not promises. The number 1 reason they lose their investment is a loss of resolve. Teams give up, lose cofounders, go [back] to school or move on in life, or lose motivation. I dropped out of MIT Sloan, for example. My cofounders had already quit their jobs. We were building Poll Everywhere, with or without YC. While I certainly want YC to be able to weed out potential flakes in the interview, I think escalating your commitment and reflecting on your past history of dedication is useful for both YC and interviewees.