Horrrraaaayyy!!!! COINS! My expert subject!!!
Ok, here we go:
You can simply, as preveously mentioned, snap (or 'click', as i say) with the hand with yhe coin palmed, which, (to the spectator) excludes the possibility of a concealed coin.
If they say 'It's in there', pointing to the hand, you say no, it's here, and pluck it out of the sky, or your armpit, or wherever you feel like. WHENEVER you just show someone a vanish (not part of a trick), always reproduce the coin quickly, magically and convincingly. That way, no-one has a chance to think about where the coin was. Even if you keep a duplicate coin in your shoe or something, so you can produce it from there. Whatever way you do it, don't (1) end a trick with a coin palmed without doing some sort of 'empty-hand manouver', or (2) without reproducing the coin. You could even do both.
This rule doesn't always apply, but in most cases, the reactions are better
and there are less hecklers.
Use the 'Downs New Changeover Palm' (Moden Coin Magic, page 318) or any other 'empty hand manouver', like the 'Warm-Up Production' (Revolutionary Coin Magic), but without the production.
Oz uses the 'Downs New Changeover Palm' on the 'Half Dollar Flipper-Coin' demo (where he vanishes the 1 coin, before making 3 again) This is my #1 peice of advice for you, as it doesn't reveal any coins, it just shows your hands empty.
You can backpalm the coin, as previously mentioned.
You could ditch the coin.
These are just some suggestions. If you don't want to use them, there's no pressure.