All air passengers traveling from the U.S. to Mexico must have a valid passport. The maximum stay allowed for visitors to the country is six months, so passports must not expire within 6 months from the date of entry into Mexico.
Air travelers from the U.S. who have passports from countries included on Mexico's "no visa required list" (see Resources) aren't required to have a visa. In lieu of a formal visa, these visitors can apply for a visitor's permit called a Forma Migratoria Mutiple, or FMM. This form can be picked up from the check-in counter at U.S. airports and visitors can fill them out before they land in Mexico. Applications for the FMM can also be acquired and filled out in Mexico after the plane lands. The fee for an FMM is $22 as of November 2010. It's customary for airlines to collect these fees for the Mexican government by including them in the cost of airfare under taxes and surcharges.
Travelers must meet certain requirements regarding their personal effects brought into Mexico. Their personal belongings in their luggage can include new or used clothes, shoes and toiletries in quantities suitable for their intended length of stay. If these are worth more than $1,000, foreign trade dues will be required at the port of entry. Additionally, each member of a family is allowed to bring $300 worth of other items. Medication can be taken into Mexico, but a doctor's prescription is necessary for psychotropic drugs.