1. Know your story.
If you don't know why you are applying for the job and how to articulate that, don't bother reading the rest of this.
2. Know the firm you're applying for and know the news.
Yes, it's easy to get firms confused and their cultures may all seem the same. But, they have executed different deals lately and you need to show that you are interested in that firm.
3. Take advantage of all interactions with employees, especially your "interview buddy", prior to your interview to get as much information on what to expect.
Many firms set you up with an alum from your school when you are preparing for your first round interview. This means you should ask all those questions you were too embarrassed or unwilling to get answered in other forums. This is an opportunity to learn what the job is really about, and how the firm selects those who advance to get the offer. For example, Goldman Sachs employees will emphasize how important it is to know how to succeed in a team-oriented environment; so when you have to answer the question "what is your greatest accomplishment," choose something that you are obviously take pride in but also demonstrates your collaboration and leadership. Every alum will emphasize something different at each firm, and try to pick up on what they are saying not explicitly but implicitly. Many times these alums don't have a say in your decision, so they are there strictly as resources.
4. Practice talking aloud when you go over mock questions.
When you try to go over the logic in your head, everything may become muddled together. Speak aloud when you are thinking of what to say, as you will know what to emphasize based on importance and how it will sound.
5. Be confident.
At the end of day, just be confident. If you have the right attitude people will come to listen to what you have to say.