If you've been keeping up then you know that a marinade is made of two parts, the flavor and the medium. The flavor is most anything you like. The medium is usually an acidic liquid to carry the flavor deep into the meat. This liquid is usually vinegar or a citrus juice. In Mexican cooking lime juice adds a great flavor and helps tenderize the meat. Lime juice is the traditional marinade for making fajitas and many other Mexican meat dishes.
When it comes to the flavor, you don't have to make it hot and spicy. Adding fresh cilantro gives your steak an authentic Mexican flavor without adding any heat. Of course if you like the heat then adding a variety of chilies, either fresh or powdered is the key. I prefer a mild chili on my steaks so I go towards the anaheim. This long, green chili is best fresh. Roast it over an open flame and chop finely. All chilies have a better flavor fresh. If you want to tone down the heat, remove the seeds and wash. Roasting will also reduce the heat a bit. If you really want the heat then try the habanero or Serrano, but be very careful.
Like most great cooking traditions, meats are served sparingly. Typically a Mexican steak will be served sliced with other things, like beans, rice, tortillas, etc. The proper way to slice a steak is straight from the grill. Place it on a carving board and, using a very sharp knife, slice at a 45-degree angle against the grain of the meat. This makes easier to chew and exposes the flavor of the meat to anything you might add it to.