How to turn down an offer if you aren't sure where you're going yet?

I'm a PhD applicant. I recently got an offer from a school I will not be attending.

The Professor that I spoke to politely asked that, if possible, I give an answer as soon I know if I'm attending the university.

I've already determined that I won't be attending, as I'm deciding between a couple other offers that I like better. What's the best way to turn down the offer without explicitly mentioning where I'm going instead (since I don't know yet).