Checking baggage outbound: Connecting internationally to ZIH via Mexico City (MEX): both Aeromexico and Mexicana (and some allied airlines) automatically forward your baggage straight through to ZIH so it arrives with you, and you go through customs at ZIH. Specify that service during your departure flight check-in, and make certain your baggage receipt indicates ZIH.
Documentation: Valid passport, of course. At airline check-in counter or on board, you are given one tourist card and one customs declaration slip. Be sure to sign the tourist card in both spaces indicated on the back. Connecting within Mexico to ZIH: you go through immigration at the first stop in the country; have passport and completed tourist card ready. Customs declaration is submitted at ZIH airport customs arrival area. Retain your baggage receipt to show when exiting baggage retrieval area. (Mexico is admirably conscientious about this verification practice; other countries are not.)
Accumulate mileage: If you are traveling with a paid ticket, be certain your reservation record includes your frequent flier number of either your carrier or the acceptable affiliate airline of your preference.
Aerial view of the Ixtapa-Zihuatanejo coastline: don't miss it, possibly upon approach and most certainly upon take-off... Have your camera ready.
Keep your tourist card inside your passport and do not lose it! You must submit it to depart the country.
Duty-free purchases homebound. Take advantage of bargains on tequila, Kahlua, colognes etc at the duty free shops at either ZIH or MEX. If you fly home nonstop from either airport, no problem. However, if you connect within the USA (e.g. Houston) to your final destination, beware: Security procedures require your baggage to come off the plane for inspection at the first stop within USA. It will then be placed on a conveyer belt to your connecting aircraft. And your duty-free purchase? You are probably carrying it. But you will be confronted with yet another security check where "liquids" are considered a security hazard and will be confiscated by the TSA guards. Solution: when your baggage comes off for customs inspection, insert your duty-free purchase into your suitcase so it will travel home with you.