What Do You Tip Your Hotel Maid?

Just about to head down to the SES San Jose conference area but left 3$ on my bed for the maid. I remember the first time I found out about tipping maids.... back 4 years ago I kind of went nuts at my first conference scoring all the free shirts I could and had them all over the floor of my hotel room. When I got back to my hotel room later that night I noticed that all the shirts were super nicely folded on the dresser with a note that said thanks! Then I noticed the 40$ or so I had left on the bed was gone. A friend at the conference told me that you are supposed to tip the maid by leaving money on the bed and she no doubt had thought thats why I left 40$ on my bed. Anyway... Curious how do you tip your hotel maid if you do at all? I used to leave like 5-10$ when I left but then I thought perhaps the same person does not clean the room every day so then I started leaving 3$ (sometimes 5 if I do not have any 1's). So what do you do?