Many fruit machines have a 'hold' button.This lets you pause certain reels during play, increasing the chances of winning. For example, if you had two lemons in a row, you could hit 'hold' and continue spinning the remaining reel in the hope of getting another lemon. Some fruit machines also have a 'cancel' button.
Modern machines are usually set to pay out a certain percentage of any money that is put into them (usually between 75% and 80%). These machines are therefore much more likely to pay out if they have had a lot of money put into them but not payed out recently.