Baste every 15 minutes for approximately 3 hours until done. Most folk broil the flank steak for fajita type fixin's. Turn when it releases from the grill. Continue until not more than rare. Also grill sliced sweet and hot peppers and onions.
Slice steak very thinly accross the grain and serve with grilled peppers.
Round steak, rump roast or sirloin tip roast will be cheaper, more tender and tastier than flank. Flank was a throw-away part until fajitas came along. It is unwarrantedly expensive because it is one of the few cuts of beef that the Japanese let in without extravagant tariffs.