So, you're lucky enough to have received an interview from YCombinator. Congratulations - you're a good hacker and now you get a free trip to Mountain View or Boston.
It's not a job interview
This is definitely not like a job interview. You won't be wearing a suit and neither will they. Nobody has arranged the chairs to make sure you feel inferior. Don't worry about whether your posture is at the right angle to indicate "interest" but not "aggression". This interview is about more important things, such as:
Passion for the idea
Passion is contagious. When someone truly cares about something and puts themselves on the line for it people always take notice. Taylor Hicks won American Idol. QED
Make sure you have an idea you really care about. Don't try to fake it - if you're not actually passionate about your idea then come up with a new one. Quickly. Your idea should show up in your sleep. I have actually had dreams where parts were on slides in bullet points. I wish I was kidding.
That said - don't mistake passion for stubbornness. You should be as excited about your idea as possible without being violently attached to it. If you're passionate about a bad idea it's probably because there's some really good morsels in there. YC will help you pick out the good parts.
Whenever somebody says your idea has something wrong with it, they are giving you a chance to learn something. The right answer is always "why do you think that?" instead of "we'll prove you wrong".
(Re-)Re-Read the application
Every question asked in the interview is hidden somewhere in your application. Think about it: they've read 400 - 500 of these things. They don't remember yours or anybody else's all that well.
Naturally then, they'll ask you things you've already covered. They'll challenge you to think critically about your idea, how to market it, who is going to use it, and they might even ask how it will make money. All of these things are on the application! Read over it again. Expand on your answers and re-think them.
Bring a demo
Whether or not you have a demo already done, you need one for your interview. Start building it now! We didn't have a demo when we applied, but we had one for our interview thanks to two weeks of non-stop coding (whenever we weren't at our day jobs). Plus, it's nice to be able to say: "Look what we did in two weeks!" If you already had a demo, code like crazy anyways. Show progress.
It doesn't have to be done, it just has to be as much as possible. It gives you something to talk about, and it's nice to show that your ideas are already turning in to reality.
Start your company anyways
If you don't get accepted to YC, you should start your company anyways. You need to believe and show them that if they don't invest in you, they will have missed their chance at being in on the next big thing. PG talks about this frequently - nothing drives acquisitions and investments quite like fear.
My mom would be disappointed if I didn't include this mantra in every part of my life. But it's actually true. I hate job interviews - they're very artificial and weird. The YC interview is a conversation that honestly is exhilarating.
Enjoy it - and good luck!