It's much easier for a magician to tell what card a spectator has chosen if the magician knows in advance what the card will be. That is why "card forces" are so popular, and why there are as many ways to force a card as there are cards to force (give or take a few).
Some of the most famous forces include:
Simple force: The cards are fanned and if the spectator tries to take the wrong card, the magician holds onto the deck really, really tight.
Riffle force: The magician riffles through a deck of cards, and when the spectator says, "Stop!" the magician keeps riffling until he comes to the card he wants to stop at.
Cross cut force: If the spectator chooses the wrong card, he gets punched in the face.
Countdown force: The magician counts down from twenty, getting angrier and angrier, until the spectator chooses the right card.
Cut deeper force: The spectator cuts to a card, but if it's the wrong card the magician suggest that he cut deeper in the deck.
Classic force: A simple force while wearing top hat and tails.
Basic force: The magician just hands the spectator a card.
There are also decks of cards called "forcing decks." These are basically just a normal deck of cards with all of the cards glued together except for the one that the spectator is to choose. Works like a charm.