Catch an employee as he or she is leaving at the end of the day, and say, “I just want to make sure you know how much I appreciate you and all that you do for the company.” Accompany with a genuine smile and if it’s your style, maybe a pat on the shoulder.
Some may react with slight embarrassment or even with an awkward silence, but that’s okay. Assuming that you’re a decent boss and the overall work environment is pretty good, they’ll walk out the door feeling on top of the world knowing that they matter.