If you have a nice phone (iPhone especially), be careful where you use it. Whilst I have no hesitation in many parts of the world talking on the phone as I walk down the street, that is not a good idea in Buenos Aires. The phone itself is a ready target, and also gives the impression that you might have some other valuables. It also goes without saying that if you're on your phone, your attention is there instead of on your immediate surroundings, so you make yourself that much easier to target.