I don't think there's a way to for her to nonchalantly accept that you're already committed emotionally to somebody else. Whether the two of you were "officially" an item or not, it's still going to hurt the poor thing!
I'm sure it'll suck to tell her, but you really don't have a choice.
I doubt you'd lose her as a friend completely, but it's very likely that she won't want to see you/talk to you all too much in the immediate future. If it was a friendship that was meant to be, it'll persist.
Best of luck bud.
I appreciate the fact that you appreciated and answered all the comments you got. Also, you're into Cowboy Bebop, which in itself speaks volumes about your great taste :D
If you were clear from the beginning that you had no romantic intentions toward this girl, then it should be as easy as simply mentioning the girl you're going out with right now. You just have to tell her, you don't have to justify anything else since you told her what you were willing to take part in from the start of your friendship.
There's no easy answer. Tell her, don't tell her, continue to see her, or don't see her anymore...whatever you do, be kind to her. You can do that, can't you?
Well, you may have to accept that you might lose her as a friend...such is life sometimes.
Ain't no thang, puddy tat ;)
I'm not offering you any "sly" methods.
Just say the honest truth. Sometimes, that's better than sugar coating something. In the end she may stop waiting on you and have her eyes open to someone else.