The magician brings out two decks of cards. The spectator gets to choose one and takes it in his hands. The magician takes the remaining deck. The magician asks the spectator to “do as I do.” The spectator performs every shuffle, mix and cut that the magician performs. In this manner, the cards are thoroughly mixed.
The magician and spectator trade decks and each cuts to a card and memorizes it. The card is replaced in the deck, which is cut again and then exchanged.
The magician and spectator perform one more cut and then look through their decks to find their selected cards and bring them out. The cards are found to be the same.
When I began in magic, I often performed "Do As I Do." It's a great trick that offers lots of audience interaction, can be performed at any time that you have two decks of cards, has lots of opportunities for comedy and the ending is surprising with an inherent build-up.
If you would like to learn "Do As I Do," there's a version that I teach here on the site.