We eat flank steak (solo, not as fajitas, etc) several times a month. Not all marinades are up to the task of tenderizing a flank steak. It needs acid, oil, and probably some salt.
I think this is the one I grew up on, but I can't find it in stores around DC:
http://www.mccormick.com/Lawrys/Flavors/Seasoning-Mixes/Tenderizing-Beef-Marinade-Mix
The Safeway generic version is a decent, but not great replacement. I think DH generally adds some BBQ sauce to it.
Italian dressing can work OK, but I'd marinate it overnight.
We also stab the steak all over with a fork before putting it in the marinade.