I appreciate my assistants and try to tell them that on a regular basis. And yeah (answering someone else's post here), they DO appreciate mentions and little letters from the company president. If it's a larger company, a little recognition goes a long way.
I'm not sure if and how it applies to your field of work and more specifically to (your) assistants, but I will generalize and talk about workers: if they are productive it means the company probably makes more money. If the company makes more money and you give them just letters and words, they will actually be demoralized. Sure, few of them will ever let you know, because they are afraid of criticizing the employer; some of them will even fake being happy. But that doesn't make them happy bees.
Letters and words don't feed you (or maybe paper does?), don't pay your bills and so on. If you want to show your appreciation reward them in a way that actually matters to them.
Otherwise, I appreciate the way you are dealing with people, encouraging them to come forward when they make mistake (after all everybody makes mistakes sooner or later) and encouraging them to be creative.