Trick Effect & Routine:
You ask a spectator to call "Heads or Tails" they do, and you give them a coin from your wallet and say here you go... spin it. They do, and find that they got the call wrong. You then say, "Yes, it was sad, but I knew you would get it wrong. You see, magicians like me ALWAYS get it right! You spin the coin and say "Heads" you are right! You spin again and say "Heads" you are right again! The spectator thinks they are catching on and says, "Hey! It ALWAYS lands on Heads!" You promtly prove them wrong, and call "Tails". You spin the coin, and it lands tails up. Amazing?

Preparation:
Get one quarter and SLIGHTLY file down the edge of the coin. Any edge, just make sure you know witch one it is. Take a second quarter and file down its OPPOSITE edge.
Mark them in some subtle way so that you can tell them appart in a pile of coins.

Method:
Bring out all your coins and say "Heads or Tails?" If they call HEADS, pull out the coin with the HEADS side shaved. TAILS with TAILS. Say, "Spin it and find out..." Then, as they are spinning it, go to put your coins away and palm the other quarter. When they get it wrong, you take it and do it. After they catch on, switch it for the other one, and spin it! You're guarenteed to get it right!