The Found Ring
The trick of the found ring is a very common scheme used by criminals in Europe, particularly in Paris. It is usually performed by a woman, but it wouldn't be unusual to encounter a man with this trick as well.
Basically, the woman drops the ring in front of you and asks you if you dropped it. When you say no, she proceeds to make conversation and try to sell you this ring, telling you it is made of gold, when it is actually worthless.
Busy Trains
Trains are a fast, easy means of transportation that can get you anywhere in a city. Unfortunately, they are also frequently used by thieves to get all your valuables, especially if they notice you are a tourist.
They attack when the train is really full and push into you while smoothly sliding their hands into your pockets. They are pretty skilled and people usually don't even notice them, so always keep and eye on your belongings.
The Distraction
This is probably the oldest trick in the book, so be careful. Whenever you are strolling down the streets of an unfamiliar city, try to stay alert whenever someone shady comes near you.
Thieves usually cause distractions so that they can get into your pockets, such as bumping into you or spilling something on your clothes. They usually work in teams, so one person will distract you while the other one steals from you, so never stop for any stranger that seems weird.