This is one of our favorite marinades. It was on Alton's Brown's Good Eats show for Fajitas. It is also fabulous if you use a pork tenderloin and cut it into medallions.
Skirt Steak Marinade
1/2 cup olive oil
1/3 cup soy sauce
4 green onions, washed and cut in 1/2
2 large cloves of garlic
1/4 cup lime juice
1/2-1 tsp red pepper flakes (to your taste)
1/2 tsp ground cumin
3 Tbsp dark brown sugar (or reg brown sugar is okay too)
2 lbs skirt steak (or whatever meat you want)
Combine all ingredients except for the skirt steak in a blender and blend until smooth. Put meat in a zip lock bag and pour marinade over meat. Squish around a little to make sure everything is coated. Put in fridge until you are ready to BBQ it.