This recipe is tried-and-true.  A “can’t-miss” party favorite.
Updated 20-Aug-14 with links to photos showing the process

1 cup teriyaki sauce
1 flank steak, 12-16 oz
two or three cloves of chopped garlic (optional)
Eight Kaiser rolls or similar hard buns

Put the flank steak in a shallow container containing the garlic and teriyaki and allow to marinate for three to six hours. You can also let it soak for several hours or even overnight if your flank steak
is frozen.

Barbecue over medium flame for seven to ten minutes, turn and cook for another five to seven minutes. (Charcoal works fine, pan-frying is acceptable but not preferred)

HERE’S THE IMPORTANT PART:

Flank steak is notorious for its toughness which is why some butcher shops score it by cutting grooves into it. DON’T use scored flank steak for this recipe as it will result in a less flavorful steak since scoring allows the juices to leak out.

The key to preparing the cooked steak is to shave each slice off at a sharp angle into 1/8 inch strips. Flank steak has a grain (just like wood) so cut across the grain to reduce the toughness (see figure). As you shave off the cooked slabs you may find that the inner part is still rare. If so, return the uncooked section to the barbecue and cook until it is done to your liking. You will also find that the shavings disappear as people stop by to pick them up and eat them.  Here’s a video showing how to make short work of the slicing process with an electric carving knife.

Another neat thing: Put the used marinade in a frying pan and heat until near boiling. Add sliced onions and cook until the onions are translucent. Use these cooked onions to top your flank steak sandwiches.

If you have leftover flank steak (unlikely) you can heat it up the next morning for a breakfast of steak-and-eggs.