The importance of really following the arch of the 'invisible coin' in the air with your eyes and head movement can't be overemphasized enough. This provides the misdirection that will make the moment you turn your wrist and cover it with the other hand undetectable.
You can have fun with your participants by asking them to guess "Heads" or "Tails". If they guess correctly you can stand in awe at their psychic ability. If they guess wrong, you can assure them with, "No worries! Even a broken clock is correct two times a day!"
