I'm going with Tin on this. I love flank steak, but you really need to marinate it.
I like to marinate in Lime juice (lemon is ok) and some garlic. Best left 24 hours, but I've had it at 4 hours and it wasn't too bad.
Here's another thing I do with flank.
Turn your oven on to bake at it's lowest temp, rack in the middle, Saute some onions, when golden brown pull them out and leave as much oil. Lightly coat the meat in proccessed chipotle (just grab a can 'en adobo' and put it in a food processor you'll only need a few ounces so buy a small can and put the left overs in the fridge) Fry that up for a few minutes each side depending on thickness. Put the meat on to the middle rack of your oven, don't forget a drip pan, leave it in there for 10ish minutes. While that happens heat up some small tortillas, get your salsa ready (almost anything chipotle based even Tabasco, verdes can be good too). Cut meat into strips across the grain, pile into tortillas, top with onions and salsa. It's pretty quick and tasty.