Don't fear the flank steak! I had to talk my bro into buying this inexpensive cut of meat for fajitas a couple of years ago... But it was a friend of mine (Katherine, sup girl!) who started me buying it. Lots of people think it's gonna be a stringy piece of meat. But if you cut it the right way, you won't have to spend extra time with the Waterpik because it won't stick in your teeth. (Promise).
Marinate the steak overnight (or for a few hours) in a gallon Ziploc bag with soy sauce and olive oil... You can add chopped garlic if you like. That's it! Then grill it for a few minutes on each side, according to how done you would like it. Let me say, however, that not letting it get past medium insures that it will be the most tender. Then let it rest for about ten minutes on your cutting board. (If you don't let it rest, the final product will be drier because the juices will run out.) Then, it's all about the cut. Slice the meat thinly, and at a 45 degree angle. Most importantly, make your cuts with your knife perpendicular to the fibers in the meat. In other words, cut across the grain. At this point, if the meat is much rarer than you would like, you can put the slices in a pan and the thin pieces will cook further rather quickly. (However, try and get it right on the grill, because it will be more juicy if you don't have to do this.)
Since I was putting these into tacos, I cut the slices into smaller pieces so no one had to gnaw as they went on a long slice. The hard part for us is making sure the meat makes it into the tacos, because we tend to eat it as we slice it!