Came back yesterday. We typically make 3-5 trips there per year. I always use a handler. And then it is as easy as the US, let them do all the work for you. If you want to do it yourself, can save some money but trade some time and headache.
You need:
First landing airport in US and Mexico has to be an airport of entry or customs airport.
Radiohandlers license for plane and pilot (never had anybody ask for it but legally required).
Customs decal to come back in the US. Can get it same day online from CBP, lasts a year
AIU (landing permit) from Mexico. Can get it where you land and is good anywhere in Mexico for 6 months.
Valid FAA registration, airworthiness cert for the plane, medical and pilots license for the pilott. Always checked.
Mexican insurance. Usually about $180/year. Theoretically if your policy covers MX it is legally good enough, but each airport has its own version of legal.
ICAO flight plan to depart VFR or IFR, no night VFR
Passport
Picture of your airplane to give to the commandant
LOPA (diagram of the seating of your plane) Can usually get a copy from the POH
US eAPIS 60+ minutes before departure
Call US 60 min to 24 hours before return to US. Unless given email confirming permission to land. (only some airports do this). If in doubt call for permission to land.
Mexican eAPIS and refile it 60 minutes before departure.
Mexican passenger manifest in Excel format. All pax and relevant information.
Copy of maintenance log book or at least a copy of the last annual. (never been asked but required)
Log of recent flights of the aircraft. (never been asked but required in a lot of countries). Possibly could just copy the last 6 months off FlightAware or similar.
Strong recommendation to use Baja bush pilots. They automatically file US and MX APIS, automatically generate and send the MX Excel passenger manifest with relevant information, and are a great resource for information. Using a handler, and Baja Bush Pilots portal, I can completely plan and file a flight in under 10 minutes. I keep a notebook with all my international paperwork and multiple copies, so don't have to recreate the wheel each time.