When you stay in a hotel, you tip the cleaning staff, at least $1 per person in the room per day. (I learned this about 20 years ago.)
You tip every single day, not at the end of your stay, because the same person doesn't necessarily clean your room every day. (I learned this about 10 years ago.)
This is the new thing I learned at International. You do not tip by leaving the money on the dresser, top of television, bedside table, or any other hard surface furniture, even with a note. You tip by putting the money on the pillow, so there is no question whatsoever that it's deliberate and meant for the cleaning staff.