In a mixing bowl, whisk together the orange juice, lime juice, soy sauce, olive oil, garlic, cumin, chili powder, black pepper, and salt.

Place the flank steak in a resealable plastic bag and pour the marinade over it. Seal the bag and refrigerate for at least 1 hour, or up to overnight.

Preheat the grill to high heat.

Remove the steak from the marinade and discard the marinade. Grill the steak for about 5 minutes on each side, or until it reaches an internal temperature of 135°F for medium-rare.

Remove the steak from the grill and let it rest for 5 minutes before slicing.

Slice the steak against the grain into thin strips. Serve with chopped cilantro, diced onion, lime wedges, and salsa, if desired.