What is an appropriate tip for a maid at a hotel?

Do you leave a tip for the maid staff at a hotel? What is the appropriate amount? I usually leave up to $5 with a thank you, but I want to be fair. Is there a rule? What do you leave?

I tip the maid according to how messy I am on any particular day. I usually leave $2.00 per day of normal cleaning and after I left a pile of sand in the tub from my shower after going to the beach I left $7.00.

I leave $2 a night per person in an average hotel. Classy hotel, it should be more.

I think it depends on the rating of the hotel, the number of guests and also how messy you are. When we go away my husband leaves $5 a day and if we need extra towels or toiletries I will ask the maid at the cart, give her my room number and slip her a ten