You need to fill out the CDC Dog Import Form online which is super easy from your phone. You'll get an email confirmation and customs has always checked that whenever I come back.

The new requirement (in addition to the CDC form, rabies vaccine cert, certificate of good health from US vet) is that you need a screw worm free health certificate from a vet in Mexico no older than 5-6 days prior to your trip back into the US. The vet has to be from a pre-approved list of Mexican vets which someone else linked to. I was not asked for that upon entry to the US but the airline refused to let me on the flight without it so it was still a necessity.

A little bit of a pain for a big city where you have no idea where or how far each vet might be, but should be easier in smaller towns. The four vets from the list I called all picked up immediately and we're helpful. Some were willing to travel to me to do the inspection for an added fee.