After reading a lot of threads on this subject, it seems like most people tip each day and usually anywhere around $2.00-$5.00 US per day.
I agree with Lexi about the gifts. You must let a manager know if you intend to leave small gifts (ie: perfume,nail polish etc.) as they will get questioned when leaving the resort to go home. At least that was the way it was when I worked at POP Village and Playa Naco.
The money is much more appreciated, in my opinion. If you leave it daily, it shows good faith and the cleaning staff will go out of their way to make sure everything is up to snuff for you.
Marlie