At one time I had read a tipping guide which said housekeeping should be tipped $1-$2 per person, per night, per room.
For the 2 of us that would be $2-$4 per night total, so I would leave $5 every morning. We always end up with lots of extra towels and a whole bunch of soap and shampoo and towel animals, sometimes a thank you note too, last trip we ended up with about 20 of each soap/shampoo which we brought home and gave to family and friends.
I think it's best to tip daily instead of at the end of your stay; if housekeeping knows they're going to be tipped regularly you're more likely to get extras.