Give them an unexpected surprise!
This is a specialty card that allows a surprise ending to a trick- you can produce a 3 1/2 of Clubs card. For example, let the spectator choose a card from your deck. You "bet" that the card you are holding is exactly half the value of the spectator's card. The spectator happily accepts the bet, as he is holding the 7 of Clubs. Lo and behold- your card is the 3 1/2 of Clubs, exactly half his card's value! For this idea, you would need to force the 7 of Spades card.
See the second video below to learn how to force the card.