Take any card in your deck and remove it from the pack.
With scissors, cut a strip about an eighth of an inch off.
To find the exact length, hold another card behind it. When the short card is cut and flush with the card behind it, you should see the numerical value of that second card.
Insert the short card into the deck.
Now you are set to perform several different tricks that use this short card as a means towards forcing a card on the spectator.
The reason it works as a force is because when you riffle through the deck, it will automatically make a louder pop and a more natural stop at the card directly behind it.
Due to this, many magicians will place it in front of a previously chosen card rather than leave it to random chance.
This way, you can start a trick by having the spectator stop you as you riffle, which seems natural and gimmick-free, even though where they stop you is exactly where you intended!