The next time you go to a party, leave a note for the host.
Say that it was a great party. That everyone was having a good time. That you enjoyed yourself spectacularly. And be sure to relate some details, like bits from overheard conversations, the reaction to a certain groove, how good the food was, and who drank way too much.
If you happen to have a Polaroid camera, bring it along, take some pictures during the party, and leave the images with the note.
Leave the note (and Polaroids) in a place where the host will find it after all the guests have left, the music has stopped, and the lights throughout the house have been turned off — perhaps near the kitchen sink. That way, one of the last things the host might see before he or she crashes is your note of appreciation.