Most slot machines in land-based casinos still use a sensor to determine how many coins or bills are being inserted. By interrupting these sensors with a pocket laser light you can make the machine think you're inserting coins or bills while you aren't.
No, there are no surefire ways to win at slot machines. These games are based on random chance, with each spin being independent of the one before it. Because it is difficult to foresee or manipulate outcomes, the best strategy is to play for fun and set a budget to avoid financial losses.