i sometimes drop a quarter while doin greg wilson's 3/4 across, and with the right patter, it actually serves for great misdirection

Dont ever let someone know what is going to happen in the trick.
Let us say that you droped a coin say "Well I have magic in my blood, But it has not got to my fingers yet."
If you are a true magician you never let them know that you messed up, a true magician will still pull it off some how. Laypeople dont know how it works just it works so even the mess up looks like part of the act.
Hope that helps.

Dropping a palmed coin isn't too bad, or too hard to come out of. I usually just look down, act surprised, and say "I was wondering where that one was!" Then I pick it up, pretend to to pocket it, and put it back into palm. If possible, I change the course of the routine that I was in to something else that doesn't require a palmed coin, and save the one that fell (that's now palmed again) for something later.

Never DENY the fact that you screw up but don't draw attention to it either.

I like "W" although i wouldn't ever do it.
Also you could talk about misdirection and say my assisstant just snuck an elephant into the back room.

This is a cut-n-paste of a post that I put up on a similar subject recently. The dropped coin line is a repeat of my last post, but the other two are quite effective:

" I still, every once in a while, will do that "doh!" thing. I have come up with a few better recoveries though, here's a couple for ya:

Me (as I very obviously and visually screw up a move): "Hmmm, did you see that?"
Spectator: "No, what?"
Me: "Then I must be quicker than I look"
-This is only done when the mess up is painfully obvious to everyone watching. If I mess up, and no one really catches it, I say nothing!)

Me (as coin accidentally falls out of palm): "There it is! I was wondering where that one was."

Me (As I lose my place in a routine, or go blank altogether): "We interupt this trick for a test of my brain. This is only a test." OR "We interupt your regularly scheduled magic trick to bring you this other trick that I didn't just completely forget how to do..."

Those are basically wordy versions of "Doh!"

i really like x just because you say a really cool word...

I usually try to get a laugh at it ... i mean , we are just there to have fun ... I usually close my eyes and start laughing , i then pick up the ocin and tell them , ok , so what were we talking about ? .. (obviously trying to change the subject)