You will notice a wide variety of tipping styles and levels from guest to guest.  There is no one correct answer for the question, "How much to tip?"  Any answer you do receive is very opinionated.  As is always the case in a foreign country though, it is best to tip in the currency of that country.  

Due to recent changes in Mexican Federal laws, it is not easy (and more expensive) for a Mexican citizen to exchange U.S. currency for pesos.  Travelers can acquire Mexican peso in any number of ways:  ordering from their bank before departure, withdrawal from an ATM machine that dispenses pesos (best to use only bank affiliated ATM machines), or going to a local bank branch (must have passport) and exchanging their home currency for Mexican pesos at the teller window.  

Going to a branch bank is the most cost effective because of the superior exchange rates offered and the lack of ATM and transaction fees.  Nearly all resorts are happy to break large denomination bills into smaller bills, however they will no longer exchange U.S. currency.  How much of your home currency to bring is up to the individual traveler, though a good average is $150-300 U.S. dollars per week's stay for a tipping budget.  If you have pesos left over you can use them on your next visit to Mexico.

General guidelines for an AI (All Inclusive)

NOTE:  The following is highly subjective.  
Most All-Inclusive plans state that the price includes ALL tips.  However, many guest do tip at some level, and for some services.  At the better resorts, you will see no change in service level caused by tipping, not tipping, or even over tipping.  I have seen instances where service has dropped a notch or two toward an abusive tipper who expects and demands better service than other guest because of the amount of tip they offer.  Most, but not all, resorts pool their tips among the entire staff or by type of staff - housekeeping, drink service, wait staff, bell boys and so on.

The best guideline to give is to do what you feel like doing.  The service professional in Mexico will appreciate any tip you might wish to offer in recognition of good service - just as they appreciate a sincere smile and "gracious."  The average visitor to Mexico is surprised at the high level of service that is offered as their normal way of caring for their guest and will want to reward it.  

General guidelines for an EP (European plan) hotel (not AI)

Unlike All Inclusive service, tips are expected, and have become part of the cost of service.  As was the case above, these guidelines are also highly subjective.

Housekeepers: $20-40p per stay, more if you have caused them extra work
Waiters: 10-15% of the bill, depending on service
Bartenders: 10-15% of the bill, depending on service
Airport transfer:  $20-40p depending on number of bags
Taxi:  5-10%

In Conclusion

There has been a lot of discussion online regarding bringing gifts for staff in addition to tipping. The general consensus has been that if the gifts don´t take away from the cash tip amount, then they are fine, but that most staff prefer cash tips to gifts. The Riviera Maya differs from Cuba and the Dominican Republic in the general standard of living and the products available to local workers. With WalMart conveniently located in Playa del Carmen, local hotel staff have access to American products at a low price.

A quick search on any of TripAdvisor's forums for "tips" will yield any number of discussion threads.  Most of them heated and inconclusive.  The best guideline always remains:  Tip what feels right to you.