Yes our customs guys can vary wildly in temperament. In Nogalas, I just hand my passport through the airplane window and he takes the Customs form (get spares when you walk in, fill them out in advance) and on I go. In Key west the Customs Agent reached and grabbed his pistol butt when I opened the airplane door, then he screamed for me to stay in the airplane.
If you want single event Mexican Insurance, you can always do the Mexicard with Macafee an Edwards:
http://www.macafeeandedwards.com/agent.htm You pay by the crossing, vice six month policy. Also, if you do multiple trips, you can get a single entry, 6 month or annual permit for flights in Mexico (this is your flight permit for flying in Mexico). Not expensive, but you need copies of everything:
CofA, Registration, Insurance, Radio Station Licence, Pilot's Licence, Medical, Radio Telephone Operators Licence. If your aircraft is owned by an LLC or Corporation, a Notarized Statement, signed by an Officer of the Corporation or LLC allowing you to operate the aircraft in Mexico along with a Weight & Balance, a copy of the picture page of your passport may be required as well. You can't have too much paper in Mexico.
You will have to check in at the ramp with the military, they just want your pilot's license number, tail number and where you left from. You then need to check in with Immigration (fill out a landing card) they stamp your passport and your flight plan closure form (you get a stub from the landing card you need to keep and surrender on exit)). Then Customs, who may want to look in the plane, have a dog sniff the plane and check your luggage, or they may just stamp your flight plan closure form and wave you on, then its off to locate the SCA guys, they record your landing for fee purposes you pay on exit. Then off to the DGAC (comandante) to do the final stamp and sign the form, which you pass on to the flight services desk, which I guess will close your flight plan, about 10-60 minutes after you land. All this repeats on exit, with turning in the flight plan being the last step again, but you pay the fees with the SCA dude or dudette (who occasionally take a credit card).
You may want to write your pilot's license number on your hand. In addition to all the forms, each stop has a clipboard that the person records exactly the same information the other person just wrote on another clipboard, with your tail number and pilot's license are required. If you say your tail number using a phonetic alphabet, you may get a deer in the headlights look from whom you give it to. Get used to saying it in spanish, even poorly is better than the phonetic alphabet. Again, while using the airport identifier would be logical of where you departed or are going to, logic is not the rule in Mexico, just say the town's name. All airports seem to be named after Generalissimo Iguana Sierra Madre or the like, the name of which nobody seems to know and they act surprised when you point to the enormous bust/plaque/statue bearing the name of the airport and a likeness of the named General. Be attentive of little details, like the ICAO identifier for Hermosillo is MMHO, while the national one is HMO, somebody is dyslectic down there. While all this paperwork seems intimidating along with folks compulsively filling out clipboards, finding scanners that never seem to work and staring at computer, apparently not getting any sense out of what they are reading. They are all unbelievably good natured about it all, even if you screw it all up. I have landed gone in found nobody, gone back tied down the airplane, unloaded, left all the bags in the customs area gone to the café and had lunch. They all tracked me down there, shared coffee and went on to do the paperwork dance.
Bring your own tie downs. Fuel early, like when you arrive. You may not find the guy when you need to leave. Always get spare forms, fill them out in advance. Keep copies of everything in a folder. If you have more paper then them, you win. Sunshades for the windows are an excellent idea, most of northern Mexico could easily be referred to as a desert. Being a desert, it is dusty, really dusty. Bring water to rinse off the dust on your windshield and some paper towels to wipe it clean. Extra oil is a good idea, you may not find it, or your special kind in a timely manner should you need some.
Just to make my life more pleasant, eApis (ours) decided that I needed to change my password, and wouldn't let me log in at the last minute. After a tense 30 minutes of poking around the web page, I noticed a link, very inconspicuous, to reset your password. You click it, fill in your new desired password, which is required to have a slew of special characters, caps & numbers designed to make it impossible to remember (mine is now something like $$MxCiptle45). You then eventually get an email with a special code to use after you log in to reset your password to the new string of gibberish. WRITE IT DOWN, with all the other stuff, like your decal number, pilot's license number and passport number and keep it in the folder I mentioned.
Always have some spare change, there will nearly always be somebody wanting to carry your bags into the airport or out to the plane, you need to tip them. You can get your plane washed while you do what you came to do in Mexico. Usually about $20-25, depending on the airport.