Don't be gentle. Be blunt and honest. Let her know that there is absolutely no way that you would ever be interested in her. I known guys where they try to be nice and let the girl down gently, but it doesn't work. You have to be straight forward and, even if you do regard her as a friend, you should consider not seeing her for a while.
It depends on the situation. If she is someone in your "Friend Zone" or platonic friend you suspect has a crush on you then simply be honest about you valuing her friendship but you don't have any romantic feelings for her. On the other hand if you have been intimate with her and have given her the "illusion" that you are an "exclusive couple" or if in fact you have been a couple but now you want to move on then you need to breakup with her. Awhile back I wrote on this subject. Hopefully it will be of some help to you. Best of luck! http://dashingscorpio.hubpages.com/hub/ABreakupMethod