Start by buying cheap forks and getting your hands on pliers.
Using the pliers, place their "jaws" in the middle of the fork.
Bend the long end six or seven times, both up and down. Don't do it any more times or it will break.
Straighten the fork out and remove the pliers.
Now you're ready to perform! Pull the fork out and call two spectators to the stage. One will hold each end of the fork.
Place your thumb and index finger on the middle of the fork and begin making a rubbing motion. As you do so, slowly bend the fork. This entire time you should be using patter to discuss how you're going to bend the fork.
Bend it two more times, thereby breaking it, and then remove your fingers to show the fork has now broken right in front of their eyes!