This is a fun pocket trick that requires little preparation.
Take an open packet of five sticks of gum out of your pocket. Tell the audience that you have removed one stick of gum and replaced it with a folded up dollar bill (Or five or twenty or higher.) Pick four volunteers from the audience and offer them a free choice of any of the five sticks of gum. You keep one stick for yourself. Now offer to trade sticks with anyone who wants to trade. Tell them to open their wrapped stick of gum as you open up yours. Peel back the silver foil to either side and push up the dollar bill where the audience can see it, saying, "Tough luck! You should have traded when you had the chance." (Or "shouldn't have," if they did.)
The Secret Revealed: Fold a dollar bill in half and then again in half twice more until you have a flat packet slightly smaller than the stick of gum. Put this is in your pocket with the packet of gum. When you pull out the gum, palm the bill and hold it between your hand and the gum where it can't be seen. After you have handed out the four sticks offer to trade your fifth stick. Take it out of the hand that holds the folded bill. When you put it, or the one you traded for, back into that hand slide it between the bill and your hand. Make sure the bill is now facing you on the side of the packet that opens. Now you open the wrapper on your stick far enough that you could pull out the gum but DON'T! Instead slide the folded bill up from below so that it appears to the audience that it is coming from inside the wrapper. As soon as the audience sees it, crumple the wrapper with the gum inside and casually put it in your pocket so that you have both hands free to unfold and display the bill. Tell your audience that you wish them better luck next time and, because they have been such good sports, they can keep the gum. Then put your bill away safely in another pocket.