Counting cards is one of the most infamous blackjack strategies. Contrary to popular belief, counting cards does not require memorizing every card that has been dealt in a game of blackjack. Rather, card counters simply keep track of the ratio of high cards to low cards that are in the deck and place their bets accordingly.
The simplest card counting systems employ easy math. These systems assign the number -1 to Aces, Face cards and 10s, +1 to low cards and 0 to medium value cards. As cards are dealt the player keeps track in his head. As the number gets higher the player knows there are more high cards in the deck; as the number gets lower the player knows there are more low cards in the deck. The strategy and stakes are adjusted accordingly. Some players like to keep a side count of Aces as well. Simple card counting systems for beginners include the Hi-Lo strategy, the Red Seven count and Knock-Out Blackjack, also known as the KO system.
Once you become a more experienced card counter you can move on to more advanced systems that allow for a more accurate count. These include Wong Halves, KISS and the Omega II card counting system.
It is important to note that while card counting is not illegal it is frowned upon by casinos. Live casinos do reserve the right to ask you to leave the table if you are suspected of counting cards.