Usually $2 a day, leave it daily on the pillow before we leave the room. Or sometimes just hand it to the maid if we're still in the room
If after the first day we receive extra good service (I don't know how to define that, it's just a feeling such as they leave extra bathroom amenities or do something else special) I add a dollar the following days. Generally I try to notice if it is the same maid each day or a different one.
Also tip another $1-2 in the evening if I request turn down service.