Everyone who has played poker or even Go Fish knows the basics of shuffling cards. There's the riffle shuffle (combining two halves of a decks and making a bridge), overhand shuffling (quickly splicing cards from the deck back into the deck) and regular ol' mixing all the cards up on a table. Which way is the best?
Math can explain! In order to achieve the closest possible shuffle to a random order there could be in a deck of cards, you have to riffle shuffle 7 times. That's a lot, right? Not really. In order to reach that same randomness with the overhand shuffle, you would have to do it 10,000 times. And if you wanted to mix the deck up on a table, you would have to keep mixing for a full minute.
So riffle shuffling 7 times is the best and most efficient way to truly randomize a deck of cards. The overhand shuffle might be the worst way (which is probably why you never see it happen in a casino) with the mixing method being effective but annoying.