Because it is such a lean cut of meat, flank steak does not have the fat or marbling to keep it tender. Marinating the steak can help keep the steak tender while grilling. Also, flank is best cooked to medium-rare doneness. Over cooking flank can cause it to toughen. Finally, by slicing the flank against the grain, you can insure that each bite is tender and not overly chewy or tough.