I usually tip about $5 at higher end hotels and $2-$3 at the lower end of the spectrum. I'll leave it on the desk or bedside table with a thank you note. I will also make it a point to mention to the housekeeper that I left her something, after finding out who will be cleaning/cleaned my room. Some hotels have someone checking the room before the maid comes in to clean.

I think the housekeepers are some of the most overlooked, under appreciated workers at a hotel.