1. Learn how to conduct an interview.
You need to understand what is driving the interviewer and how he or she is thinking. So know enough about the interview process to put yourself in your counterpart's shoes. If the person is bad at interviewing, you can run the show. If the person is good, you have to figure out how to meet their agenda, make your points, and still be likeable.
2. Learn from other peoples' mistakes.
The best way to see people making errors in interviews is to interview them yourself. But you can also read about other peoples' interviewing incompetence. Jobaloo shows how candidates misread a seemingly innocuous question. And CareerBuilder lists some examples of extremely bad judgment.
3. Know your agenda.
What is the image you are trying to convey in the interview? Match that to the kind of job are you trying to land. You should have three points about yourself that you aim to get across in the interview. Before I became a full-time writer, mine were: great at executing a plan, a manager who everyone loves to work for, very reliable. I wanted those points to come across because I wanted to be hired to a position where I would have a lot of responsibility to execute a visionary plan and manage a large team.
You can find tons of lists about how to interview well. Take a look at them and you'll notice that they are all about practicing: Avoid too much information, cut the puffy stuff, know your strengths and weaknesses. These are all things you can practice. If you think you can wing it in an interview, you're wrong. There are no questions that cannot benefit from preparation, so any question you look unprepared for makes you look clueless about the interview process at best and lazy at worst.
5. Be comfortable with silence.
People who can remain calm during silence look powerful and comfortable with themselves. People who have to fill silence end up saying stupid things. Part of your interview practicing should be to sit, saying nothing, so you are comfortable when that happens in an interview.
The interesting thing about preparing to ace an interview is that most aspects of preparation carry over into the rest of your life. When you know who you are, and how to convey that to other people, when you are comfortable with a pause, and you are good at reading another person's agenda, you will function better in all aspects of your life.