Carne Asada Mojo or Marinade

I lived in Guatemala for a while many years ago, and learned that Guatemalans love carne asada. 
I honestly never really understood what that meant since the translation is: grilled meat.  I mistakenly thought this meant any meat and couldn't figure out what the big deal was.  Many years later, living in Colorado with lots of friends from Mexico, I finally learned what carne asada really means.
Super thinly sliced skirt or flank steak.  Perfectly marinated, and "just" cooked on the grill so that it is tender and juicy. The meant is served in a corn tortilla with refried beans, rice, pico de gallo, guacamole, & sour cream.

The easiest way to get the best meat for carne asada is to look in your area for a 'carniceria' (literally - meat market) ask them for 'aguayo' meat - this is the best for carne asada.  Then ask them to add their 'mojo' or 'marinade'.  They'll usually put the meat in a bag, add some marinade, and then you can let it sit in the fridge overnight or for a day or two.

If a carniceria isn't an option - Costco often sales carne asada (already in a marinade), or you can purchase thinly sliced flank or skirt steak and do your own mojo or marinade.

I usually have to slice my meat into halves or fourths just to get the thinness I need.
Here's a great recipe for the marinade/mojo (which tenderizes and adds great flavor)

Carne Asada Marinade or Mojo
1/4 c. key lime juice (the very small green limes)**this is key for the flavor
1/2 c. olive oil
2 T. white vinegar
4 garlic cloves - pressed or very finely diced
1/2-3/4 c. chopped cilantro
1/2 t. ground cumin
2 t. kosher salt (less if table salt)
Jalapeño or seranno chiles, minced (optional - to taste)
1/4 t. crushed red peppers (optional - to taste)
2 t. sugar (more or less to taste)

Using a fork, pierce meat, and place in non-metalic (reactive) container.
Cover with marinade/mojo, and turn.
Allow to marinate for at least 15-20 minutes, then turn and marinate on other side.
This may be refrigerated overnight, but bring to room temperature prior to grilling.

Heat grill, and cook for 2-5 minutes each side (depending on grill), so that meat is cooked, but still tender.
Place on a platter and cover with foil, and allow meat to rest so that the juices flow.
Note that the acids of the vinegar and the lime help 'cook' the meat during the marinade.  Also, pureed kiwi can be added to the mojo as it contains meat tenderizing enzymes.