For our family of four, I tip $4.00 per day for housekeeping. We have not taken advantage of the turn down service at WDW, but at other hotels, I tip $1.00 if I am traveling alone and $2.00 if the family is with me. In larger cities (NY, Chicago, SF, LA, etc.), those figures should be doubled.
I leave the housekeeping tip each morning when I leave the room. When I return to "freshen-up" for dinner, I leave the tip for turn down service.
To insure that the tip is received, I either place the tip in an envelope with "Housekeeping" on the outside, or I fold a piece of the stationary in half and write it on the outside with the tip inside the fold. I do this because I once left the tip for housekeeping just laying on the night stand, and it was still there when I returned. The house keeper must have thought I forgot to put the "change" in my pocket before I left.
I budget the tip as part of the room cost. Just as I budget tip as part of a table service meal.