Flank steak is a great inexpensive cut of meat to grill. There's a couple of keys to tender flank steak. Don't overcook it and marinate it overnight before cooking. I am always looking for new marinades for flank steak. I found this one on Simply Recipes. It's easy and uses ingredients you may have on hand.
Grilled Marinated Flank Steak
1/3 cup olive oil
2 cloves garlic, minced
2 Tbsp red wine vinegar
1/3 cup soy sauce
1/4 cup honey
1/2 teaspoon freshly ground black pepper
2 pounds flank steak
Freshly ground pepper
1 Score the surface of the steak with 1/4 inch deep knife cuts, about an inch apart, across the grain of the meat. Combine the marinade ingredients. Place steak and marinade ingredients in a large freezer bag. Coat the steak well with the marinade. Seal the bag and place in a bowl. Chill and marinate for at least 2 hours and up to overnight.
2 Using olive oil soaked onto a paper towel, coat the grill rack of your grill with olive oil. Preheat the grill with high, direct heat. The grill is hot enough when you hold your hand about an inch over it and you can only hold it there for about a second.
3 Take the steak out of the marinade bag and sprinkle generously on all sides with coarse salt and freshly ground pepper. The salt and pepper will help form a savory crust on the steak. Place steak on the hot grill. If you are using a gas grill, cover the grill. Grill for 4-6 minutes on each side. Half way through grilling on each side, turn the steak 90° so that you get more grill marks.
4 Flank steak is best eaten medium rare; well done will make it too tough. When the steak has cooked to your preferred level of doneness, remove from the grill and place on a cutting board. Cover with aluminum foil to hold in the heat and to keep the steak from drying out, and let rest for 10 minutes.
5 Make very thin slices, against the grain, and at a slight diagonal so that the slices are wide.