I discovered another advantage to using a card guard -- it made simple deck switches much more effective.  After doing a card trick, you re-case the deck, put on the special card guard, and pocket the deck.  A few tricks later, you take out the deck again, remove the guard and go into another trick.  But it is a different deck with a duplicate card guard.   The simple addition of the card guard -- the act of repeating the taking it off the card case when the deck is re-introduced -- creates an unconscious connection in the spectator's mind.  A sort of retention of memory/logic that this is the same deck.

Of course, card guards can be expensive, so having a duplicate for your deck switches can add up.   But I found this card guard at a great close-out price, so I'm offering them in sets of two.  And the price is less than most single card guards.

See the photos for the design -- a grinning devil on a Bicycle-like back on one side -- the other side has a stylized Ace of Spades and mirror writing of the word "Devil".  The edge has a prediction of the AS SPADE.