I've only stayed on property once - i'm a spoiled local girl, but whenever i've stayed other places I normally leave a couple bucks everyday in an envelope on the dresser and label it "To our Housekeepers (Mousekeepers)...Thanks for all you do". At the end of the week I leave a slightly larger sum with a nice thankyou card.
The reason I tip everyday rather than just once is that I may have different ladies everyday, so i don't want to just tip the one that happened to care for our room on the last day
I tip more the last day because there is more involved in getting the room ready for the next guest
I would always rather ere on the side of tipping to much rather than be cheap...but thats just me