Place the prepared flank steak fillets into a large resealable plastic bag.
In a mixing bowl, combine the olive oil, lime juice, orange juice, red wine vinegar, Worcestershire sauce, minced garlic, salt, and brown sugar, if desired. Stir until thoroughly blended.
Pour the marinade over the flank steak in the bag. Seal the bag securely and shake gently to ensure the meat is evenly coated. Place the bag in the refrigerator and let it marinate for a minimum of 2 hours, or up to 24 hours for optimal flavor.
Heat your grill to a very high temperature.
Remove the marinated steak from the bag using tongs, allowing excess marinade to drip off. Lay the steak on the grill and brush lightly with BBQ sauce, if preferred.
Cook the steak for 5 to 10 minutes on one side, then flip it over and brush the other side with BBQ sauce. Grill for an additional 5 to 10 minutes. For rarer meat, limit grilling to 5 to 7 minutes per side.
Check the doneness of the steak, and if necessary, grill for a few extra minutes to achieve the desired level.
Remove the steak from the grill and apply another layer of BBQ sauce if desired. Allow the steak to rest for a few minutes before serving.
Serve the steak with your choice of accompaniments, or slice it into strips to use in tacos.