There are 3 tricks to keeping flank steak tender. First, don't cook flank steak past medium rare. Second, once it's cooked, let it rest for at least 5 - 10 minutes so the juices stop running and go back into the meat. Finally, slice the meat against the grain, not with it. Slicing against the grain shortens the muscle fibers and allows the meat to fall apart and melt in your mouth when you chew it.