I also don't agree with the "dress under the radar" comments, wear what you want but just pay attention. Knock on wood I've never been robbed successfully here or when I lived in Barcelona (although I had many more unsuccessful attempts in 6 months in BCN than in 3 years here!!).
One good tip maybe for you guests that you could do, is to provide them xerox copies of maps rather than a map itself. I worked at a hotel in Barcelona and our concierge often gave guests just xerox copies so that they could peruse the map freely in the street without the huge colorful tourist map that calls so much attention. The guests really liked that idea and I think it helped a lot.