It is one of the most dreaded moments in a person's life. No one can get over rejection quickly, it is best to just brush it off and move on. But how did you get here in the first place? Where did you get your signals crossed? How can you prevent this from happening again?
All too many times a guy jumps the gun and asks a girl out and gets shot down embarrassingly. Worse than this is the guy that will not asks any girl out even if she is hitting on him so hard he has bruises. So what is the happy medium you ask? Think! Yes Think and Watch. If you have a girl being friendly to you watch how she acts around other guys. If she acts the same to these dudes chances are she is just an overly friendly girl. If she acts much more friendly to you then maybe, just maybe she likes you. If a girl likes you, she will let you know. Girls are used to guys getting the wrong signals so they will make sure to let you know that they are interested.
My favourite display of idiocy was when I was out at a restaurant with a friend of mine. The waitress passed us and he leaned over to me and said, "She likes me." After asking why I figured out that my so innocently, naive friend,(let us call him Naive Steve), didn't realize that the waitress was being friendly to him to get a good tip. And trust me she was, I know him. If he had watched her with other male customers he would have realized this.
Online Dating saves us this embarrassment. If you message a girl and she doesn't answer you back move on to the next. No embarrassment, no big deal.