It used to be that when I received a Fav, I would journey to the user's page and leave them an appreciative comment. These days I just swing by and drop them a llama, because, you know, why not. I used the think the former option was the most appropriate, until I realized that I'm not really expecting any comments when I Fav work myself. Plus I'm fortunate to have Watchers that fav several of my deviations, and I'd hate to be spamming them with comments, even if they are thankful.
How do you guys thank people for Favs?