Do you mean how to project male presence in general or at times when you're being affectionate with your girlfriend in public? I don't have much advice on the former, I was never much good at it, but the latter - well, you have one of my ex's to thank for this. She actually told me to be more assertive and more spontaneous (well, what she actually said was "Jesus, you're more of a girl than I am!"). I asked what she meant and she said that the guys she'd been out wth (except for me, lol) were very spontaneous, like... they'd be walking down the street and he'd just pull her close and kiss her, out of the blue. Or put his arm around her and draw her closer. I was usually just passive, and let her do all that. Not because I wasn't interested or anything, it just somehow didn't feel right to take the lead, as it were.
Not sure how much of that is a purely masculine thing and how much a confidence thing, as Brandon says, but yeah.