Key tips, to avoid standing out from the locals in Rio:
If you're a guy, and you have a 't-shirt tan', get rid of it pronto - even if you have to use fake-tan or something. No guys in Rio have t-shirt tans, and me and my friend used to laugh at the 'fresh meat' walking along the beach, blatantly just arrived! If we noticed, the professional thieves had an easy job! If you really REALLY want to blend in, buy a pair of speedos - seriously! All the Brazilians wear them...! (I realise that's extreme!

)
If you're female, try and copy the clothes style of the cariocas...bright clothes, short skirts, and unfortunately the non-existant bikinis - any size, any shape, one size seems to fit all....or not...and can be bought on the beach

Money tips - girls, that's what your bra was made for

alternatively, a very attractive small carrier bag

, can be helpful as a disguise. Guys - shoes

Thieves are professional, and they'll look for signs of money-belts etc. The above is no guarantee, but I lived there for 5 months, and had nothing happen to me. Another good tip is to avoid wearing jewellery that looks expensive (e.g. a watch) which acts as a magnet to thieves.
On the beach, avoid taking anything expensive in case you fall asleep in the heat. At night, avoid the beach (unless you're with a large group and have absolutely no valuables on you if you want to hit the beach bars)
The only time I ever heard of anyone getting mugged in Rio is if they did something silly themselves (e.g. deciding to go to sleep on a ledge in Lapa! True story x 2!) Don't miss Lapa, it's fab for the Brazilian Baile Funk, but don't go to sleep there either!