Handwritten notes are one way to show individual appreciation. Take the time to think about what they sacrifice personally to help you.
An individually thought-out gift is also a great idea. Think about what their interests are and tailor your gift to that. Once piece of advice is to not give out a standard “business logo” gift to everyone. This takes away all personal ties to the gift and can make an employee feel neglected.
One of my bosses bought our staff lunch, and for a poor college student, it meant a lot and showed me that he went out of his way to appreciate me. My old boss sent me a package of my favorite kind of holiday treat, Bear Hug Fudge. When my employers show appreciation to me, it makes me want to serve them more and give my all to work.