a lot of you have commented on safety concerns in Mexico- I have been all over Mexico on several different trips, I will put your minds at ease a little.... Most of the murder/kidnappings are drug related, and for the most part occur near border towns.... things really have blown out of control there, but cancun/riviera maya area is not the main concern. With that being said, obvious things like staying with someone else, not walking down dark streets, etc.... Just like any other place in the United states you might travel.
You can also go to USA.gov: The U.S. Government's Official Web Portal and look up travel advisories for different countries and areas....
Happy traveling!