Freezing the deck
Among Canasta players, there is one action that makes most very nervous. Some avoid it outright, while others tiptoe around it, using it only with caution. Others are foolhardy, willing to take action again and again, regardless of the personal risk. This action that strikes terror into the heart of Canasta players everywhere is freezing the deck, and it worries them for good reason; a frozen deck is often the kiss of death to a game, where the player who can freeze and unfreeze the deck to his or her advantage wins, and the player with less control over the frozen deck loses. There is nothing to fear from this tactic, once you know how to use it to your benefit.
What is 'Freezing the deck'?
In Canasta, "freezing the deck" involves placing a wild card (a deuce or a Jack) onto the discard pile, so that the deck can no longer be picked up at will; a player now needs two of the top card in their hand and must meld them when the deck is picked up. Players avoid this tactic because it can easily backfire, leaving them in the lurch when they end up giving half the deck of cards (and all of its value) to the opponent after discarding the wrong thing. If done correctly, however, you will not only shut your opponent out, keeping him or her from gaining the cards in the deck, but you will also be able to wait until the right moment to gain control of an impressive number of points.
Freeze the deck early. Ideally, you want to freeze the deck before you have melded, and after your opponent has put down their first melds. This means that they will have fewer cards in their hand, and you will have more, giving you a greater stockpile of cards to match with the top card. Many Canasta players like to meld as soon as they can (a habit that seems to carry over from Canasta's origins in Rummy 500), likely out of a fear that they will be caught with too many points in their hand. This isn't really a concern if you freeze the deck as soon as they meld, however, because your opponent will be so limited in the cards that they can draw that they will be very unlikely to ever form a canasta (and therefore cannot meld out).
Freeze Early... Not Often
Do not freeze too often. A well-executed game of Canasta should never have the deck frozen more than twice; it is very unlikely that you would benefit more than twice (if you're playing properly), even in a game that uses the entire deck.
Freeze, then stop melding
Once you've frozen the deck, do not meld at all until you've taken the entire deck. The entire purpose of freezing the deck is building it up so that you can steal it from your opponent and stockpile points. Keep gathering cards and discard only what your opponent has already melded or is clearly not holding onto (i.e., what they keep discarding).
Irritate your opponents
Keep in mind that skilled deck freezers are not overwhelmingly popular in the Canasta world, and that this kind of gameplay is seen as particularly vicious and cut-throat. Mastering deck freezing is the key to never losing a game of Canasta, but keep in mind that if you never lose, your opponents may start looking for other competition.