The easiest marinade is a bottle of Kraft Zesty Italian salad dressing. Don't use the low-fat or fat-free because they have added sugar, and the oil will help keep the meat from sticking to the grill. Marinate it all day in a ziplock bag, then drain and discard the marinade. Cook it only to medium rare; overcooking London Broil or flank steak will make it too tough to eat. Also cut across the grain so it's not stringy.