FBMG Smithy said:
But most of all, don't look scared/nervous or act/look like a tourist and you won't appear to be a potential target.
I would also add that the more you appear the dominent "Alpha" type the less you are apt to be chosen as a target.
Maybe. Americans tend to carry themselves more "aggressively" than Europeans do, even when not trying to look dominant -- our body language is part of what makes Europeans think we're arrogant. I think trying to look "Alpha" will make you stick out. If you look tough enough, it may also help you be left alone, but if you don't quite pull it off, it'll just mark you as a stranger, and a target.
In Europe, i think the best approach is to slump and look bored. Even if they make you as an American, you'll look like an American who is familiar with the area. I've spent many months in various parts of Europe, including Paris, and after a little practice I found it very easy to blend in. You know you're doing well when people (natives, not Americans) ask you for directions, as though you're the local.
In any large city, anywhere in the world, the other advice given here is good: Keep your wallet somewhere hard for pickpockets to get (the front pocket is good, unless your pants are quite loose) and stay in busy, well-lit areas to avoid being mugged. Carrying a dummy wallet isn't a bad idea, but honestly if I'm being mugged the money in my wallet is the least of my concerns. Credit cards even less so -- as long as you report the theft promptly, your total loss will be zero. Also, if your wallet is so light that the mugger guesses it's a dummy, you're likely to piss him off. Not good if he has a force advantage.
I carry plenty of cash, and I don't carry a dummy wallet. I do keep a credit card and ID hidden away so I don't get stranded if my wallet is taken. In cool weather I put it in an inside pocket of my jacket. In hot weather I put the cards in my shoe. When possible I leave my passport in the hotel room safe, especially in hot weather when there's really no good place to hide it.