On the way out, two firemen in their yellow overalls were huddled at an outdoor table over a laptop. I walked past, and then though about it, and doubled back. I thanked them for being firemen, and told them that my house had burned down once and it would have been a lot worse had the fire department not been there almost instantly. I thanked them for doing an important job and really helping people.
So be sure to thank public servants. They do dangerous jobs for little reward - the occasional expression of gratitude is not too much to ask.