In response to a question about the best way to cancel..
Thank you for offering me an invitation to interview at your school. While your offer has received serious consideration from me, I regret to inform you that I am unable, at this time, to attend. Please keep in mind that I have been offered to interview at xx highly qualified schools this year, and am unable to get to all of them. My inablity to attend your interview in no way reflects upon the quality of your program. Please do not allow it to impede you on your goal of filling your class with other qualified applicants. I wish you success in raising the standing of your school in the U.S News rankings in the future.