I understand what you mean, its because when you do like the guy, you are trying to impress him and make him think your great, you can try to hard and end up making yourself look stupid instead of flirting lol
When your flirting with guys you dont wanna date its because your more relaxed that its easier, your not trying to get the guy to fall for you!
So.. i guess when your trying to flirt with a guy you like,pretend its someone you dont wanna date or try not to look at him too much! That way you will forget how gorgeous he is and will flirt better lol