Get a flipper coin. They aren't difficult to find if you know where to look. Magic shops usually carry them, and several places online sell flipper coins. They usually come in half dollars or quarters, which is important to know when you plan to pair it up with other coins for certain tricks.
Learn how the coin works. Using a flipper coin is simple; you simply unhinge it to make it look like two coins and then flip it back together to make it look like one.
Act it out. Hold your thumb over the hinge part of the coin so that it doesn't look suspicious. When you flip it, do it quickly so that no one sees the trick coin in action. With a little practice, you should get good at fooling onlookers.
Tell a good story. One of the best flipper coin tricks revolves around a narrative. You tell the audience that you have two coins, but they're too heavy to carry, so you're going to magically place one in your pocket to give your hand a rest. Then flip the coin and pull out a second, regular coin out of your pocket.
Have spares to show to the audience. When you use flipper coins for magic tricks, the audience may be skeptical. So switch out the flipper coin for a regular coin and let the audience check it out; they'll never now you made the switch.