I agree with these guys. Its a very fun place to be, but you really have to be careful. Try going away from the city to other places.. like if you are into the beech, try Vitoria, Bahia, Floranopolis.. or my favorite- Porto Segundo. For night life, Sao Paulo is probably the best in Brasil, and Rio has the best day life. Try the food in Minas Gerais (Belo Horizonte is the biggest and most easilly accessable city there.) You have to try the BBQ.
If you look American like me, you will be noticed, and you are a prime target for theives. It's usually some gang members working together, and you wont know whats going on until its too late. Dont go to Futbol games without some people who you really trust. Don't wear Puma or Nike, instead wear old shoes. (Wait 'till you see the prices of shoes in Brasil, youll see why they have to rob you!) I dont want to scare you away from Brasil, as its a great place to be, and most of the people are very welcoming. Where are you commuting in from? I try to go twice a month so I'm pretty well versed on getting there from anywhere in the US.
The woman are generally very friendly, and I suggest learning how to Samba. Sounds funny, but it works.