As Mystery states, one of the characteristics of an alpha male, is smiling, but lets Not forget the eyes!
Eyes are of the most important features in people, by looking at someone in the eye, you could judge what mood they are in, or what kind of a person they are. This judgment is NOT 100% true, but is a judgment based on observation and girls use that alot!
By having an eye connection with a guy in a club, the girl decides if the guy is a creep, pervert, fun, caring, nice, sad and ... person! So what eyes need to show?
If you are going for a night out, no matter how big of a smile you have on your face, you're eyes need to top that in order for that to be believable!
Keep your eyes wide and happy. Make it shout playfullness to the max! When you make eye contact with a girl make her feel your energy, your good vibe and that you are there to have fun and not to perv and that will get them attracted!