I also do this always, they are the hardest working people around, and like you said, it is sometimes a thankless job. I leave the money on the nightstand or wherever the notepad/pencil is and write "housekeeping" so they will not think you just forgot some cash lying there. And....on occasion, have received back a nice thank you note in return. Also, if staying while, they know who you are, and I suspect you will get their best work.I also always leave a few bucks, generally three, on the hotel bed for the maids who clean up my hotel rooms. My mother taught me that. Most maids in hotels are hard workers, and it is a thankless job, on the whole.
During the Rodney King riots we stayed at a large motel occupied by mostly police officers. Like always, I got stuck on the 12 hr night shift. After being at the motel for a while and seeing almost no maids (having no conversation/interaction) due to the hours we worked, our rooms were always cleaned at odd hours so I would leave the tip before I went to bed. One morning I couldn't sleep and after a few hours I got up and was going down to eat. As I passed a supply area where several maids were congregating, one said "good morning, are you going to be out of the room for a while". I said I was going down to eat. Later, when I returned to my room, it had been cleaned and refreshed. I always wondered how the heck did she know which room was mine?