1. In a large glass bowl, whisk together the balsamic, Worcestershire, brown sugar, and garlic.
2. Add the skirt steak to the mixture, cover with plastic wrap and let sit for 25 minutes at room temperature. Turn the meat in the bowl several times during the marinading process.
3. Prepare grill to medium-high heat and oil the grate or prepare a grilling pan for the steak.
Remove steak from the bowl and discard marinade.
4. Sprinkle both sides of steak with salt and pepper, then place steak on grill, close lid, and cook for just 3 minutes, flip and cook another 3 minutes or until done to your liking.
5. Remove steak from grill, cover LOOSELY with a foil tent, and let the steak stand at room temperature for 5 minutes. Do NOT skip this step.
6. Cut steaks into very thin slices on the diagonal and serve immediately.