I often get mistaken for a porteno, people keep asking me for directions, where buses are going, what time it is, the names of local streets, and so on. It has to be mostly about look.
Adding to the previous advice, I think the key to looking like a porteno is to act like a porteno, and to do this, channel the feeling that you have already been robbed, several times over, each year, by taxes, by inflation, by the three jobs and bosses that don't quite pay you enough to live the life-style you deserve. hopefully you already feel like that.
So next you just act as though you have nothing that is worth stealing, and anyone who needs to steal from you must have such bad taste or be so desperate that they are welcome to have your almost worthless stuff, which will include a cheap cell phone, maybe some bad jewelry and some bit of cash, because as a savvy porteno you have left anything of value at home.
The attitude of "I've already been robbed this month, so don't bother me with it again", should give you a kind of worn exp<b></b>ression of bored survival, yet proudly almost defiantly presented in whatever look you are wearing, in your face, posture and clothes.
Looking around at what people are wearing, I find I can see whatever I am looking for somewhere. Clothing here looks to me like a mashup of various under-30's fashion subcultures from many large cities in the world over the last ten years, done as knockoffs, with low-quality fabrics and assembly, slightly dated and worn with comfort, confidence and fun, almost an experimental necesity. A mixture of any metro grunge punk or retro styles works, and it can be matched badly or well. It should be clean and pressed, but not too expensive a look, not minimalist, not ascetic, and not have haute couture fashion references in any way. Although you see such references, it takes a real porteno to work them in. If you wear a daypack, it should be well worn, splitting at the seams, and dirty - pink or black are popular colors.
The body language of keeping your shoulder bag or day pack in front of you, and the way you hold your phone when texting, must be habitual and look careless and inconspicuous.
Lastly, how to walk on the street takes time to get used to. The main idea is to "conserve momentum" You keep going, keep moving at a steady pace. You go around anything or anyone larger, like a family or mob. When someone steps into your path, or when someone steps out of a doorway backward in front of you, just divert around them a bit, don't waste energy stopping or pausing when you can keep moving. You don't have to miss them by much ... or act surprised. Just keep looking where you were heading, and looking around enjoying things, being relaxed.
I laugh at warnings not to wear a gold Rolex - is it a joke? Isn't it in bad taste to own a Rolex, let alone a gold one, or to wear one outside of Monte Carlo? A wrist watch as such is almost an anachronism...
