When should "send" the girl
If you feel to the girl feelings, but at some point you want to spend time not with her but with friends or alone, you should gently explain to her that you really value your relationship, but some time will be busy. Say you'll call her as soon. Most importantly do not be rude, otherwise you risk to lose her because of this stuff.
If the woman you stuffed in girls, and no sympathy you feel, then you will really have to turn her down. Not worth the pity to build a relationship with the unloved.
There are rare cases when the girl herself is trying to pursue an intimate relationship. She's acting very frankly and makes all sorts of allusions. Better do not start sexual relations with her, if not then plan to be there, remember that "free cheese is only in a mousetrap".
How to reject a girl and not hurt her
Never forget that in any situation you have to look decent. Even if the girl strongly enforced, don't tell her bad words and not raise her voice.
When the girl offers the guy his candidacy for the role of his beloved, her excitement is through the roof. Crudely denying it, you may always be lowering her self-esteem. Instead of a direct failure, it is better to lie. Tell me it's not ready for a serious relationship or what your heart is, unfortunately, already belongs to another. Be sure to tell her: "I'm Sorry it happened, I'm sorry." She will still be hurt and offended, but not so much.
Don't postpone the conversation for later. You need to immediately explain to her the situation, otherwise, even for a short period she may believe that the relationship between you is possible, then it will be even harder to understand that it is not.
During a conversation, be extremely serious. Look her straight in the eye, so she can trust you. Finally, compliment her. Tell me something good, for example, "it's nice to know that such a beautiful girl paid attention to me" or "Trust me, not every girl dare say."
If you need to break up with his girlfriend, it is unlikely that all will turn out so just. To break the current while the relationship is much more complicated than those that did not started. In this case, you have to say is really the truth. Be sincere, your ex needs to know why you can't be together.
Importantly, in any situation remain a man. Set your priorities and be only with those who have real feelings.