The blackjack basic strategy gives the player the best advice to play based on playing continuously. At some points in your game you will be losing, but at others you will be winning. It is designed to give you an overall winning margin, however there is no saying which point in your game that will be.
Because of this, experts came up with a more difficult, but more effective strategy to follow. This strategy is card counting and takes advantage of probability again. As the blackjack game is played and cards are dealt from the deck, the deck is no longer complete, so the probability of a certain card being drawn changes. Knowing when the cards remaining are more advantageous to the player and enables different tactics and betting opportunities.
When there are more high cards like 10s and Aces, the player can make bigger initial bets because he will know that there is more chance of him getting a blackjack.
Another option is to double down during the game when there is a high possibility of good cards being dealt for you.
The system of card counting is not as hard as it seems. Rather than remembering which cards have been played, a point system is assigned to cards of different values as they are dealt from the dealer. The score tally will show when it is the best time to place bigger bets. If done correctly, this method can actually give the player an edge over the casino. It could range from between 0 and 2%.
Card counting is a very controversial strategy, but mental counting is not actually illegal. Only counting with electronic equipment is illegal. However, even mental card counting can be frowned upon by some casinos because it is losing them money!
To reduce / stop card counting, a lot of casinos have introduced more that one deck. This makes it harder to count and predict when it is the best time to bet.
There are variations of card counting that are more complex, but are designed to beat multiple decks. One of these methods is Shuffle tracking, which will not be discussed here.