We have 2 we've done. The first is similar to Rebecca's - except we really, really like Newman's Own Balsamic Vinaigrette. (His "original" flavor is prettty good, too!)
We haven't used that one, though, since I got the recipe for the Flank Steak Marinade from Boma at AKL. This stuff is awesome!! I use it on ANY kind of steak, not just "flank".
FLANK STEAK MARINADE
from Boma, AKL
Yield: enough to marinate about 1½ lb steak
¼ c ginger
½ Tbl thyme
2 Tbl garlic, minced
¾ tsp turmeric, ground
2 Tbl sugar
¾ tsp crushed black peppercorns
¾ tsp cardamom, ground
¼ c soy sauce
½ Tbl coriander, ground
½ Tbl Worcestershire sauce
Mix all ingredients in a large bowl. Add flank steak and marinate for 24 hours.
[NOTE: This is very thick more like a paste than a normal marinade.]