I leave my tip on the pillow. Since it would be unlikely to accidentally leave money on the pillow, I had heard that this was the proper place for the tip. I also have a book with me and read every night. If I finish the book, I leave the book under the money on the pillow so that they will know it does not need to go to "lost and found".
I do tip daily to insure todays cleaning person gets the tip.
Bob