I've only 'harvested' a few and so far I've been lucky and haven't been caught (although at least one croupier has spotted me, just smiled and looked the other way!). My advice, for what it's worth, is try and play a table where there are a few players, or where they use automatic sorting machines, and only take a couple of chips.

If you get stopped when you leave, just say that you wanted them as a souvenir and that you didn't realise that you couldn't keep them, hand them back and try again another time!

Goos luck

Jason