Thefts are, unfortunately, extremely common in Buenos Aires. Anywhere you go, you should be mindful of the valuables you carry. The following also applies if you're strolling about Corrientes or taking a coffee break with a friend; but specifically when taking public transportation: colectivos (buses), el subte (subway), or even taxis, watch your belongings.
I don't have enough fingers to count the number of Argentines (and those who have lived here long enough to know the culture) who have told me to be careful on public transportation.
The gist is this. On colectivos and on el subte:
If you have a purse, it better be one with a long enough strap to sling across your body.
If you have a backpack, wear it backwards so it's in front of you.
When boarding or waiting to board, watch yourself and the people around you.
When stepping off, watch yourself and the people around you.
Make sure you know where all of your belongings are at all times.
It's better to share one with someone you trust rather than ride alone.
It's better to choose a younger driver. (Why? Because older drivers have years of swindling tricks under their belts.)
Make sure you get the right change back.
Make sure your change is real money.
As a general rule, don't put money in your back pocket - or in any pocket on your back side where you can't see it and/or feel it - and don't get it out while you're riding.
You'll want to take pictures of la gran ciudad. That's great. However, you'll need to be careful every time you decide to whip out your camera - especially if you have that fancy Nikon D5300.
Know the areas you visit beforehand to know just how vigilant you'll need to be. For example, Barrio Once is notoriously dangerous. Everyone knows to avoid La Flor at night. These are the kinds of things el peregrino should know for general safety.
I'm sure all of this applies generally to any bigger city. This is my first time living among such a large number of people, so I'm a little overly-worried and over-cautious... But that's better than getting robbed, right?
Suerte, amigos --