I go to BA a couple of time every year and by following some precautions have never received any fake currency.

First: Try to get most of your money when you arrive at the airport. There is an official bank, that has a window, just before you exit the sterile area. You´ll get a great rate, and make sure to ask for lots of small bills. I change $400 usually, and get at least $150 in 5-10-20 peso notes. They may not be thrilled about it, but it will save you grief in the long run.

Second: Seperate your money into larger bills and smaller bills. I keep my large bills in my shirt pocket, and my smaller ones in my pants pocket. This also helped me when they tried to pickpocket me on the subway. I didn´t see them go into my pants pocket, but did when she had her hand in my shirt pocket.

Always try to give taxi drivers correct change. They will pull a switch on you, so it doesn´t hurt to say, 10 pesos as you´re handing them the bill. They will try a switch with a 2 peso bill and tell you that you´ve given them the wrong money, or that it´s a fake. One thing I also do, is look at the last three numbers of the bill I´m handing them, in case they try the fake scam.

Most medium to larger sized restaurants take credit cards, but don´t depend on it. I´ve seen a number that have had their machine broken for months. Some smaller places that I like, like El Yugo, only take cash.