Or you could offer little prizes while they work. I work at target and sometimes while we work, my bosses will have us compete in little mini games, like scavenger hunts, or who can get the most recognition from other employees by the end of the night. stuff like that. the prizes arent the greatest, but a little bag of candy or something from the dollar store is ok too. just something to show that we are being recognized for working hard and having fun while doing so. employees like to be praised for the things they do well, and when they get praise it makes them want to work harder for more.
Compliment or praise them when they do a good job.. It sounds simple I guess, but it works on most people. Makes them feel appreciated and like their efforts are recognised.
My Old man.. always told me that he always worked his hardest when his boss offered a prize or whatever for the person that finished first or work the hardest.
Or so something like employee of the month and whoever gets employee of the month gets a $100 bonus that month.
Positive reinforcement, reward them for doing good work (praise, little gifts, etc)
Offer like a $500 bonus every 6 months to hardest worker.