How do you gently but clearly communicate to someone you’re not interested in them?

I had an experience recently where I had inadvertently led a woman to believe that I was interested in getting to know her, when I was simply trying to be nice. She then proceeded to try to find all kinds of activities we had in common, so we could get together beyond that evening.

My question is this: once I’m already in that situation, how can I communicate that I’m not interested in going out with her, in a way that isn’t mean? I found myself simply deflecting all her attempts, and left feeling awful, yet not knowing what I’d do differently. That’s why I’m asking you all for your creative solutions here if I ever find myself in a similar situation again.