Most guys who flirt tend to do so from a position of stress, concern, and if they find themselves rejected, with anger.  When you talk to a girl she can pick up your ‘vibe’, just as you can pick up the ‘vibe’ of other people.  The last thing you want is for someone to pick up the vibe that you are a serious or stressed out person, or, ultimately, a person who is angry and unlikeable when they don’t get their way.  You can avoid all of that by approaching the whole task of flirting as a fun and playful activity.  Always remember that even the girl that rejects you might be the girl that ultimately takes an interest in you if you respond to her rejection with a smile and a joke.