How did you do it - by phone, email? What did you say? Is there interview cancelling etiquette?

I want to cancel this interview I have scheduled in November because the more we think about it, we don't want to live there, and although it is a good program, it has weaknesses in certain areas that are important to me. But I'm assuming that's not what I should tell them if they ask me why...

Thanks!

I did my cancellations by e-mail like this:
<Program Coordinator's name>,
Thank you for the opportunity to interview at <Program's name>. Unfortunately, my
circumstances have changed, and I am no longer able to keep my
interview with your program on <interview date>. I apologize for
any inconvenience this may have caused you.
Thank you,
<World's greatest EM applicant;) >

Most programs e-mailed me back thanking me for letting them know in time to schedule another applicant.