This is a good question - many answers have been talked about. The ones that surprise me are the ones that state to ditch it when nobody is looking, that is like saying - do the top change when nobody is looking...not great direction. Anyhow, many great answers, here are my experiences and thoughts.
I know many guys that hold out as long as they can – I have seen guys do card tricks with coins palmed from the last effect, to later ditch it on the replacement of the box to their pocket. I know some guys that will stay dirty for as long as they can, because to a pro...we all know that when the hand dips...and there is NO reason for it to dip, that something was stolen or gotten rid of. We may not know what...but we know it!
AND – perhaps because of this:
http://www.youtube.com/watch?v=_qQX-jayixQ
Maybe so do audiences.
But, I think with audiences, it is simple logic. Think of your pocket as a curtain...if a guy was dipping in and out of a curtain...you would start to ask what is going on back there?! Clean up does not often happen on the impact of the effect, actually, that is the time to look the cleanest, as far as I am concerned...even if you are as dirty as mud! People will start to ask “how soon after” and the ditch lets them off the hook, but if you remain confident...and they see you aren’t trying to “get rid of anything”, it will kill them more, as they themselves remove that as an out. I think sometimes we stop trying to outthink people once the effect is over...the show starts before a card is selected, and does not end because it is found.
I think going to your pockets for no reason at ANY time, even during the reaction, creates a subliminal tell. I was at a show recently a guy got accused of “trickery” because he reached with both hands behind his back to pull down his vest that was riding up. He really was just straightening his clothing...but he was called on it...and yes, it was after the trick, right after impact, at a larger formal show. I digress, so by going to your pockets, you have created an out for an observant participant. Now, certainly not all will question your hand dip, but again...for those that have read me, I don’t want to fool the easy ones. I want to fool the guy that thinks – you know what, the only way that torn and restored card could be done is if there are other pieces of card!? I believe, through testing times to ditch in my own career, that if the ditch is not motivated and/or (yep, and/or) done too soon...it can and will create suspicion. Using proper motivation, nobody remembers or questions you “putting your hands away” but if you go in and out for no reason, they will. If you go south too quick, they will. If they suspect a method in the effect, and you say, “well, how can that be”? They will say – it is in your pocket. Then what? Are you going to argue it is not, when it is, and the reason they thought that is because they saw you go there? Really, this is an awkward conversation that can be avoided by holding out longer or finding the right moment to ditch naturally and unrushed, as a few have mentioned (magicman_1999).
Too often as magicians we worry about being “dirty”, but really? Did anyone understand the concept of dirty or clean magic, before they were magicians? Sure, when creating a set, I don’t want to have to deal with the dirt (reset of stack, card reversed in middle, cards taped together) mid act...but if you can do your show and worry about the dirt later...do it. If you are palming the “evidence” and during the effect they were hid there, why do you think they can’t remain hidden for a bit longer? GUILT eats at us; it is why...I think we feel it is like the beating heart in the Tell-Tale-Heart by Edgar Allan Poe, it just drives us nuts so we want to confess or hide the evidence...when really, that same method got us to that point.
Now, I don’t think we need to hold out for long periods all the time, but I do think to be deceptive we have to understand our moments better...even create moments for ditching. This takes structure and thinking no your behalf – as well as an understanding of when the effect is over and people have stopped looking...NOT when they are distracted. As many know when they were purposely, even with a laugh, distracted...but when THEY themselves give up, it is far more deceptive a time.
I conclude with this – perhaps another slap in the face to those that defend coin magic. How can we worry about ditching something we hide in our hand, when we claim to vanish things and show our hands empty so well to create the idea OF empty hands?