In a medium bowl, whisk together olive oil, soy sauce, red wine vinegar, Worcestershire sauce, minced garlic, brown sugar, smoked paprika, black pepper, salt, and crushed red pepper flakes (if using) until well combined.
Place the skirt steak in a large resealable plastic bag or a shallow dish. Pour the marinade over the steak, ensuring it is well coated. Seal the bag or cover the dish and refrigerate for at least 2 hours, preferably overnight for maximum flavor.
Preheat your grill or grill pan to high heat. Remove the steak from the marinade and let any excess drip off. Discard the marinade.
Grill the steak for about 3-4 minutes on each side for medium-rare, or until it reaches your desired doneness. Use a meat thermometer to check for an internal temperature of 130°F for medium-rare.
Remove the steak from the grill and let it rest for 5-10 minutes before slicing against the grain.
Garnish with fresh cilantro or parsley if desired, and serve immediately.