If the issue was personally significant to you, tell them so and how. The thought currency among legislators is in concepts and anecdotes. If you can provide a genuine tale of how their action affected your life, it may stick in their minds, and may even come out later when they are trying to persuade others. If the official's action took some special courage or effort, let them know you appreciate that fact. As with any communication to an elected official, the letter does not have to be long, and must be limited to one subject only.
Some people make a religion out of sending letters to the editor.
Generally, they're not worth the time it takes to write them. However, if you already have a letter written, you might as well send it to the paper. Even if no one else reads it, if it gets printed, the elected official will see it and appreciate your effort. This can be a general event, an event related to your issue area, or even an event totally unrelated to your issue. An unrelated event could be anything-environmental groups, nurses association, retired military officers, chamber of commerce, anything.