Think Of Paris by Jacob Pedersen

You select a spectator. You show a stack of business cards. You show that every one is different. The spectators see that she can choose between everything from: color, city, country, persons, items etc. Now your chosen spectator chooses one, and after some mindreading, you not only reveal what catagory she has chosen (a city), but also the exact place (Paris).

This can be adapted in any way you like.
A powerful principle that I am using in my own shows, and it always baffles the audience.