When I am out with certain friends, as a group we get a lot of interest (sometimes to the point of it being rather irritating). Other friends, we don't get any. It is strange really because all my friends are beautiful and great people (to me!) but there are differences in the way different groups look and behave.
The ones who get more attention overall tend to be the ones who put more effort into dressing well, the ones who follow fashion and invest in nice clothes, make up, hair etc.
I have one friend who is sometimes stopped by men who have just seen her in the street (it's happened a few times when we've been in busy areas of London). They're usually creepy men you wouldn't date, and it's actually a bit annoying and invasive, but I have wondered why they do it to her and not me. I don't really think she's unusually stunning, but she dresses well, wears nice make up and smiles a lot.
So I think it's a whole combination of factors - how you look, how you dress, age, AND how you interact, body language, how much you smile etc.
Are you asking because you want more attention from men? Because it's not always a good thing!