From what others tell me, I tend to leave things a bit messy, not dirty, just a little disheveled. I don't put things away; I untuck the bed completely and don't pull the covers back; I turn the iron off, but leave it out; etc. So, I tip $5 a day - more if they go above and beyond. They work hard and deserve it.
One this is I don't do is "Mousekeeping" envelopes. My first solo trip, the housekeeper wouldn't take it because it didn't say Housekeeping. She was afraid she'd get fired for taking something that didn't belong.