Elastrickery is one of the few decent tricks around which use a rubber band. This free trick is actually being sold elsewhere on the net. The big advantage with a rubber band trick is of course that you can always carry your props with you and if you forget it you can be sure that most houses will have one lying around somewhere.
Wrap the rubber band around your first two fingers and pull it backwards as in Fig. 1
Now clench your fist, making sure that the band wraps around all of the fingers at the back of your fist as in Fig. 2.
Your audience should of course only see the front of your knuckles as in Fig. 3.
To make the band jump to the other two fingers, simply open and close your fist in one fast action. The band will appear to have jumped as in Fig. 4.