The magician approaches a stranger and speaks with her just long enough to get a sense of her personality. This accomplished, the magician takes out a piece of paper and a pencil and writes a note that is given to the stranger. The stranger is then asked to count how much money is in her wallet, and the amount of money is found to be identical to the amount written on the paper.
Simple as can be -- Derren Brown always performs this effect after performing Taking Your Wallet. Before having an assistant give the wallet to its rightful owner, Brown counts how much money is in it. Because people are so happy to get their wallets back, they never bother to remember what Brown looks like and treat him like a stranger when they meet again ten or fifteen minutes later.
Note: When Derren Brown performs this effect, he is often wrong by a dollar, likely because his legal training at the University of Bristol skimped on mathematics.