I always leave the tip for the housekeepers (both the daytime housekeepers and the evening housekeepers) tucked inside the little card with their names written in it. (I have never not found a card in the room on which they have written their names down, introducing themselves as our housekeeper). I then leave the card with the cash tucked inside out on the center of the bathroom counter. I think this leaves no doubt that the money is for the housekeeper. At least, we've never come back to find the money still there. lol
P.S. I know all about the "Mousekeeping envelopes" and I think they are a cute idea, but it's just not something I've ever done.