You probably already know this, but...
Beware of pickpockets, seriously. These guys can be exceptionally creative over there.
My first night in Peru my pocket got sliced open by a razorblade artist. Luckily I didn't carry anything in my pockets. I didn't notice anything though, and mind you, I was being very alert. They do it lightning fast, as you walk by, and then they pick up your stuff from the street.
Also beware of other tricks, such as people coming up to you with headphones to let you listen to some music. It's all a distraction from the other guy who's emptying your pockets.
Also, don't carry a backpack on your back in busy places, carry it on your chest.
Don't wear any expensive rings or watches whatsoever.
The smartest thing you can do is leaving all your valuables in a safe place in your hotel (or wherever you're staying), and just carry a small amount of cash with you to get you through the day. So if you get pulled into an alley and you have to surrender all your stuff under the threat of a big knife, all they'll get is $ 10. So don't go carrying around your moneybelt with all your important stuff in it, only when you absolutely need to.
That said, SA is great! I hope you'll have a great time.
