1 bottle of beer (I used Corona this time, but Negro Modelo works well, too)
3 limes, cut up and juiced
1 1/2 tablespoons salt
3 tablespoons olive oil
optional (teaspoon pepper)
Combine all of the ingredients and place in plastic "zip-lock" bag. Add the meat, and store in refrigerator for about 45 minutes, turning occasionally.
Before grilling the meat, shake off the excess fluid and salt the meat as you would if there were no marinade at all! Really. I think flank steak tastes best cooked medium rare to medium.