Marinate it in your favorite blend of oil and acid (vinegar, citrus, whatever). and herbs and spices. Cook it hot and fast. Cut it across the grain -- the cut is more important than the cook. It will cook fast over high heat, which is how you want flank steak to cook. So, your preparation will have a lot to do with the flavors. Oil, herbs and acid are the key ingredients in your preparations and they will matter a lot. Don't be shy with any of them. Your flank steak should be well lubed and seasoned before it hits the hot grill because it has a lot of connective tissue but not a lot of fat. Hot and fast, pay attention.