Any veterans/service folks care to comment?
It's really tough, I won't lie to you. Walking up to a total stranger and saying something from the heart, to me at least, is very unnatural.
Usually I ask them a bit about where they are stationed, what they've been doing, whether or not they've been deployed. Just small talk. But before I leave I try to make physical contact -- patting them on the shoulder usually -- and say something like "Hey, I just wanted to say thank you. You guys are doing an awesome job."
Every time I've done it I get the weirdest reaction. At first they're totally convinced I am going to say something ugly right afterwards. Then they look confused for a couple of seconds. Then it's like the sun opens up from behind clouds: it's really, really cool. Makes you feel good the rest of the day (and I'm sure it makes their day as well)
The reason the Russian army isn't in western europe is not due to the heroic effort of the Queen's Cavalry - it's due to the ability to knock two pieces of Plutonium together.
It's the nerds/geeks at Aldermaston/LLNL/Los Alamos/ Sarov/Zheleznogorsk that have kept the world safe for you.
You can't imagine the morale-boosting effect it has to simply have strangers come up and say "thank you?"