Tip everyday because you may not get the same person. I always tip $1 for clean people, $2 for messy people. My mom and I are very clean people, so we tipped $2 a day. My brother and his family always had a room that looked like a tornado hit it due to a 4 year old. He tipped $1 each for him, his wife, and a 2 month old, and $2 for the 4 year old for a total of $5 a night. We also tip more on the 4th day when they change the sheets.
Also tip more if you make a special request. There was one night when my niece had an accident, so the sheets had to be changed. He tipped $10 that day.