The truth behind appreciating can be a simple thank you. It's not "old school" or even "old fashioned" to be polite. Simple words such as "thank you" or "please" show respect and appreciating. Some will go behind the "thank you" and tell you why or how you are appreciated.
The truth behind the appreciation is the art itself. When an act of kindness or helpfulness is just that -- to be kind or helpful -- not to expect a debt or something in return, is the benefit of hearing "thank you."
Recently, I reviewed a book for an author and he sent me a "thank you." I get a lot of "thank you" emails from authors, and sometimes I don't get any thanks (not that I'm looking for it) but his email touched me, deeply. He wasn't just appreciative of the review, but of the words within the review. That made my day. For someone who is deeply connected to words, and tries to "paint" a picture with words, appreciation and thanks can really make a person's day, esepcially when it comes at a a perfect moment.
Any moment is perfect with a "thank you" or a "please." It is sad that in this day and age (and I'm sounding old now) that it is more suprising when someone is kind rather than when someone is unkind.