I bring $5.00 US bills to leave daily for the housekeeper and we leave a larger tip on the last day. Also, I highly recommend getting to know your housekeeper(s) by name and chatting with them a bit.
We have friends in the staffs of all the places we stay and are always happy to see them again. A tip and some acknowledgment go a long way. Too many people take the housekeepers, and security for that matter (we tip them too), for granted.