h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I totally agree with this article...bartenders and 'maids are absolutely wonderful, hardworking and incredibly under-appreciated people. I'd just like to also suggest that, along with tipping them and/or buying them a drink, talking to those valiant servers of alcohol is a very nice way to show them that they are indeed appreciated.
Of course, one has to be considerate when doing this. Pestering them isn't a good approach. Don't try to strike up a conversation when they are obviously very busy. Instead, take for instance the staff at your regular pub/bar. Do you know their names? Bartenders are reputedly amazing listeners but, in my experience, rarely are listened to.
I may just be babbling, but I think that the best way to show someone that they are appreciated is to show that you consider them a human-being and not just some drink-bringer. In the quiet moments at the end of the evening, introduce yourself, ("Hey, by-the-way, I'm ")and ask them how they are. Maybe even buy them a pint. ;o)