The trade between Mexico and the US is regulated by the North American Free Trade Agreement (NAFTA) that provides the free flow of merchandise (via customs) of many goods and services. It states that all custom tariffs should be lifted by 2008.
You may need a NAFTA certificate of origin ( issued by the exporter) because it identifies the materials as being covered by the terms of the NAFTA, and makes the shipment eligible for reduced duties. - http://www.fas.usda.gov/itp/Policy/NAFTA/rulesof.html
In Mexico, you should deal with an application for export/import ("pedimento") and must be registered as an exporter with the Federal Taxation authority of Mexico "Hacienda" (Secretariat of Hacienda and Public Credit ). In additional based on exported articles - you need to check if a license is required.
To export more than $1,000US - you must file for export with a customs broker licensed in Mexico.
You may find information about any specific article here - http://www.apectariff.org/ It is also published in Mexico's Duty Tariff Book.
For additional information on importing to the U.S. - contact the Trade Commission of Mexico - the World trade Center in Los Angeles, California - 350 S. Figueroa St, Suite 296, Los Angeles, California 90071 -(NNN) NNN-NNNN
The U.S. Census Bureau's Foreign Trade Division website can be used to classify products in the Harmonized System - http://www.census.gov/foreign-trade/schedules/b/index.html. If you need help classifying your product, call the Census Bureau at(NNN) NNN-NNNN For current US import tariffs - http://www.usitc.gov/tata/hts/index.htm