What a good question. I am local but I look very foreign and around 2 years ago during November I experienced 1 attempted theft per week- (thankfully i really dont have much to steal
- though they finally did manage to get a hold of my ipod in the last attempt, and it happened while I was distracted talking on the phone).
I was so frustrated that I actually contemplated dying my hair to stand out a little less- then I realised the attempted thefts had began right around the time when I started using some borrowed headphones for my ipod (mine had broken) and they were very visible. As soon as I went back to a new understated pair, all atempts of theft suddenly stopped, so you definitely do not want to be flashy with your technology or money. (I walk with ipod in pocket, my phone is an old relic and I dont use my laptop in parks or any other open public spaces, I dont use it by the window in cafes either, and if I take it out of my house it is in an everyday backpack, not in a laptop case...)
When you`re on the subte put your back pack frontwards so that you can hug it and see if any nimble hands try to open one of the zippers. When you`re on the bus, the typical moment at which pickpockets work is when you`re getting off or when its crowded. Watch out for those that stand too close, or a classic pickpocket giveaway, those carrying a sweater over their forearm.
Other advice I can give you is to move confidently and relaxed and pay attention when you`re on the street and transport. Don`t be afraid or embarassed to ignore or say a firm no to anyone on the streets who asks you for something (a coin, a light, whatever), specially if there is something about them that makes you uneasy- (better to come of as a jerk than an easy target).
Avoid eye contact with sketchy subjects.(unless you feel you`re being pickpocketed on a bus, then you can glare at them and they will probably put their hand away).
Have a radio taxi number handy, and a charged cell phone at all times, have coins ready in case you need to take a bus, and have a guia T (your public transport ally) so that you dont get lost and have to ask the wrong people for directions. If you feel threatened try to go in a cafe, bar, kiosko...
As teenagers we were taught to take our money in our shoes, so if you want to take extra care you can allways carry 20 in your shoes just in case- its a little too much for my taste but it could come in handy in case of emergency.
I mean sure precautions are smart, be aware, but don`t be paranoid, there are plenty of kind and nice people in BA too, and not everyone will be looking for the chance to take your wallet.