This has been discussed in other threads, as sort of an OT thing, so I wanted to give it its own thread and link to this article I found very useful:
http://www.gringogazette.com/southern/october31_2005/page7/
aumedina
Jun 2, 06, 8:52 pm
In general we tip 10% of the bill in restaurants, 15% if the service was really good. You should tip more in places frequented by americans, like Cancun or Los Cabos. In more traditional places like San Luis Potosi or Queretaro the expected tip is usually smaller.
Like the article said, you don't tip the cab drivers, and don't expect an envelope for the maid in not-so-expensive hotels. You can however leave your tip under the pillow (and so you assure it's really the maid and not her supervisor who takes the tip).
If we stay for a week or so, I usually tip the maid directly at the beginning and the end of the week, and always receive very good service.