Usually something quick and easy, but nothing beats the more elaborate treatment from Guerrero described by Diana Kennedy, if you have the time. My version of this is done without careful measurement of the ingredients:
Several dried chiles, usually ancho and a little guajillo depending on how spicy I want it. Chiles are stemmed, slit, seeds and veins removed, and soaked in hot water until soft.
In a blender jar, combine the soft chiles, about 1/4 cup white vinegar, peppercorns, cloves, cinnamon stick, Mexican oregano, small tomato, salt, and just enough water so that everything can be pureed.
Coat thin steaks with the paste and place in refrigerator, uncovered and allow the paste to dry. Grill and enjoy!
Bill/SFNM