Justin Kan, the founder of Twitch, a Y Combinator alum, and a long-time partner, says you have to get these three things right:
(1) Know what you are doing. You have to convince the interviewers that you are a great founding team, primarily by projecting confidence, and showing your expertise in the industry.
(2) Understand the small and the big idea. You should be able to convey the reasons why you are going to attack and dominate a niche area with your product, and then use that as a platform to successfully enter a much bigger arena. Expansion can happen in many ways, often by expanding geography, product category, or target customer. The important part is that you make a bold claim about how you will grow into something much bigger once you dominate your core niche.
(3) Convey founder synergy. More often than not, startups fail because of the founder conflicts. You have to show that you know how to work together, have clear areas of expertise/focus, and are able to resolve conflicts without sinking the company.
1. Find people who have done it. Y Combinator network is large and strong, and you should have no problem finding founders willing to help you.
2. Talk to them, and write down concise answers to their questions, and practice. You don’t want to think on the fly and have to worry about saying the wrong things.
Be concise. Get straight to the point, while making sure you cover all the important points, while also maintaining partner's interest.
Convey your “3 things”, the points that you want partners to remember about your company. Even if the conversation sways in other directions, it is your job to find a way to bring those key points back into the discussion.
Don't be defensive, and don't use jargon. Make it easy to understand you, and show that you can take and evaluate criticism. Have a lead speaker, but also enable the rest of the team to answer some of the questions.
Lastly, be confident. Remember that you’re the expert in your business. You know much more about the space than the partners interviewing you. Walk into the room knowing that you understand this subject better than anyone else.