It's not something I do often honestly. Not because I can't speak to people but I just don't want to bother someone minding their own business, or make someone worried that they are being creeped on. When I do I try to be respectful and complimentary (I love your dress/ shoes cool tattoo where'd you get it etc). If it's at a Cafe I might ask what are you reading (if they have a book), or what are you drinking/ eating? I obviously know well enough not to just run up to someone and say wow you're so hot, or use a cheesy pick up line. But I would genuinely like to know from women how do you want a man who finds you attractive to approach you in public?