Make a habit of being nice to receptionists, secretaries, assistants and anyone else you may meet when you arrive for the interview. A positive impression on these people might not help, but cannot hurt. You may get lucky and find a grad school interviewer who highly values the advice of an outside person with whom you have come in contact.
If you absolutely cannot get to the interview site, don't be afraid to suggest an alternative. For instance, if you cannot scrape together the airfare to fly across the country, inquire as to whether the interviewers will be at any closer alternative site any time during the interview period. Do all you can to wrangle an interview at some other location if it is impossible for you to get to the official interview locale.
Practice for the grad school interview by engaging in a mock interview. Get a friend or an instructor to help you prepare a series of questions most often expected during the interview. At the very least, mock interviewing can help you polish your general interviewing skills and help bring attention to problems like fidgeting, stumbling over words or getting a dry mouth.
Use the interview to provide evidence of your communication skills. It is not unusual for a brilliant scholar to be unable to translate that knowledge into easy communication. Showing that you are both academically well-prepared and able to translate that knowledge easily through conversation can give you a real boost as an applicant.
Part of the point of the interview is to ensure that you are a suitable candidate for their graduate program. The best way to prepare for this aspect of the interview and to use it to your advantage is to educate yourself about the cultural environment of the program itself. Learn all you can about what type of student excels within the program you are attempting to enter and transform yourself into an example of that model student.
Show off the fact that your education is well-rounded. Don't be afraid to work your other interests into the interview by way of using them as examples. Point up any part of that well-rounded aspect of your personality that shows you as a leader with a vitality that extends beyond classroom work and study.