I know this is an old thread, but when I fight, I mainly stick with the circle button, alternating the three others. Circle will punch your opponent in the face, while the other buttons will punch him in the ribs, block, and grab.
It also depends on who you're fighting with and against. Don't have Paul Kariya go against Derek Boogard.
Yeah, first of all, the guy you're fighting with has to be tough; second, just press a bunch of buttons randomly - thats what i do. The circle hit is most effective but they're usually good at blocking it, so you need to be quick.