Write a simple and sentimental sentence on a small piece of paper that states anything from: "I love you" to "Hey! I would love to see buzzards pluck out your eyeballs under the hot desert sun!" Then take that piece of paper, fold it, and slip it into his or her pocket for them to discover later on. It's a small, simple, and sweet gesture that will go a long way if repeated on a somewhat irregular basis.
Caution: Do not be alarmed if you don't get immediate acknowledgement from their part...the small note can easily be lost or discarded without the realization of its intended purpose. The true acknowledgement to this romantic gesture is when you discover a little note in your pocket...you just have to wait for it.