Its a mental thing when it comes down to it. It all has to do with self-image. How do you think about yourself? What kind of man do you describe yourself as? Myself ... I'll tell you right now I describe myself to be the "Lone Ranger" ... I don't bring friends with me much because really my friends can be a hinderance to my game (They will hit up on any women I talk to) ... So I go alone like the Lone Ranger ...
Anyways enough about me ...
Who cares what people think (I certainly dont) ... I'm talking about what you think of of yourself. I think theres a place for this kind of thing and you can check it out ... "Inner Game" ... its here on this website and you should definately check it out.
Now how you present yourself to people...
Try looking up pics on google like "James Bond."
See how he stands at the bar with his martini in hand ... notice how he holds himself up ... Thats how an Alpha male holds himself in public when he's alone ... and I try my best to emulate that (lol I dont drink martinis but you get the idea)
If anything just dont slouch with your arms cross or anything (thats a closed off posture and makes you unapproachable) ... arms to the side ... shoulders back and chin up!!! Or take up some space ... lean on your arm with your hand resting on the bar wide and away from your chest ... and your drink somewhere in front of your chest ... or sit on a chair with your leg resting on your knee/lower thigh with one hand on your drink on the bar and the other resting on top of another chair (I do it lots and I get tons of looks like "who does this guy think he is" type looks haha lol)
I hope this helps with what your looking for ... but definately check out the "Inner Game" forum ... Cheers!