"No trouble here" is a fine way to indicate that you are not looking for a fight. On a more complex level, in my opinion, aside from preplanned muggings or things of that sort, fights usually start around emotions connected to shame and humiliation. Many males, perhaps most of us, are acutely sensitive to being publically mocked or shamed and the upsurge of anger that develops in such a situation can lead to an inevitable explosion...this is especially true when there is an 'audience'nvolved. Said "audience" may be no more than one person, a girlfriend, for example, in front of whom one is loathe to be put down or humiliated.(In the setting in which I work, a psychiatric hospital, it is not uncommon for a patient to begin to escalate toward an explosion: staff members know they have to set firm limits, but it has to be done in a manner that allows an 'honorable' out, so to speak..."you are getting too worked up, sir. You need to go to your room for a few minutes then we will talk. I cannot listen if you yell or threaten me.", for example). The situation would be worse if there were an audience of other patients to whom the person feels he must play, or in front of whom he feels he cannot back down.The same is true in the street. So what is one to do? If a punk is showing off his toughness in front of his friends, about the worst thing you can do is taunt back, or wisecrack... that would obviously up the ante and move the confrontation closer to the physical level. "No trouble here, mate" and keep walking is obviously the wisest course. Remind yourself that "sticks and stones may break my bones but words can never hurt me" as they hurl jeers at you while you walk away.In a one-on-one situation, no audience present, the humiliation factor is less pronounced... so you just have to avoid making threatening or humiliating statements yourself so as not to escalate the situation.Body language is important. If you are collapsed, shuffling your feet, looking down and acting very submissive you may actually be inviting an attack. Obviously, if you are red faced and yelling and raising a clenched fist you are asking for a fight.If you stand strong and firm, shoulders down, hands in front of you and open (interestingly, the universal "let's calm down" sign, palms down, patting the air in a back-off gesture, is also a very good 'ready'posture in case the attack comes).
Of course, all of the above presumes that we are in good emotional control and not looking for the fight. The hotheads among us, who number very few, I hope, might see it differently.