It's a little different for every relationship. Though a few standard angles usually get the job done here:
If you're not in too deep, easiest is to just tell her "Hey, it's been really great. But I feel like if I let this relationship go on too much further, I'll be leading you on. It's time for us to part ways." Then just stick to your guns when she tries to negotiate.
If you are in deep though you're going to have to go with "You're an amazing gal. You're going to make some guy super happy someday. But this just isn't what I need. And the longer we let this go on the harder it'll be to do what we have to do."
She'll be upset, because it's the rare breakup that isn't upsetting to one or both partner. However, better to nip it in the bud early than to wait until both parties are more invested and it's even harder to do.