Here:
http://forums.jetcareers.com/threads/flying-in-mexico-stamp-land.212116/
What airport are you gunning for? If it has an FBO, TAKE IT.
If it doesn't you'll need ARROW documents, insurance policy and your license and Medical. Try having both originals and photocopies of everything. If you're doing a one time flight, after you land go to the Comandancia and show your documents. They'll give you this bank deposit sheet which you pay at any bank and once you have the receipt, turn it in for the permit. It costs around 800 pesos which is around 50 USD.
Check also that you have also consulted Airport operations (landing and parking fees), Despacho (FSS), INM (Immigration), SAT (IRS) and SAGARPA (EPA) before you can officially finish a flight.
If you plan to do multiple flights into Mexico, ask for a Multiple Entrance Permit. You'll need the exact same documents plus a letter to the Comandante requesting the permit, where you describe your Name, Aircraft type and model, colors, and serial number, owned by whoever your Registration says. Add a passenger list of whoever MAY be brought aboard in the plane, too.
Once you request the permit, you'll be given 2 bank deposit sheets, one for the first arrival, one for the year permit. Each is around 50 USD. Once you knock 'em boh out, turn in the receipts for the Permits.
If you need ANYTHING drop me a line. I'll be more than happy to help
Take Care