Stefan Mandel's strategy for winning the lottery involved buying every possible combination of numbers. He calculated the number of combinations for a given lottery and only targeted lotteries where the jackpot was at least three times the cost of buying all the tickets. This ensured a profit even after covering the cost of the tickets. To buy all the combinations, he raised money from investors and then purchased every possible ticket combination. This method worked for Mandel several times, but modern lottery systems have implemented changes, such as limiting ticket purchases and digital monitoring, making it almost impossible to replicate his strategy.