An article at Concierge.com talks about some common street scams and provides defensive tips which might be useful for infrequent travellers.
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Some comments on those scams:

The “baby” toss (Italy)
– The scammer throws you a “doll” so that you drop your bags/belongings trying to save the “baby”. Another guy will then try to snatch the bags. It may seem impossible to work, but the thief might succeed in a crowded place.

The phony parking-lot attendant who breaks into your car.
– We are not quite sure if it matters if its a parking lot attendant. Basically the car can get broken into by anyone if the place is not guarded. The only difference with a fake parking-lot attendant is that you will lose that extra few dollars for his “service”. The car is destined to be broken into regardless if the phony attendant exists.

The camera snatch
Someone takes your camera and helps to take your picture, but he will not return you the camera unless you pay him money. Such a brute force approach is more of a daylight robbery than a scam. The tip given by the writer is correct, just shout “Police!“. I will simply give an extra hard kick to the groin before doing that.

The crowded subway car
– The usual pickpocket that exist probably since the invention of common public transport. Everyone should be familiar with this, except that it happens quite frequently on certain rides and country.