Before we stayed at AKL last year, I always marinated my beef in some Newman's Own Salad Dressing - either original or balsamic viniagrette, preferring the balsamic. We really enjoyed the flavor it gives the meat.
However, since our trip last May, we now use Boma's Flank Stead Marinade. It's awesome!
FLANK STEAK MARINADE
Ginger 1/4 c
Garlic 2 Tbl
Sugar 2 Tbl
Cardamom, ground 3/4 tsp
Coriander, ground 1/2 Tbl
Turmeric, ground 3/4 tsp
Thyme 1/2 Tbl
Crushed Pepper 3/4 tsp
Worcestershire Sauce 1/2 Tbl
Soy Sauce, low sodium 1/4 c
Mix all ingredients in a large bowl. Add flank steak and marinate for 24 hours. This should be enough for ~3lbs of steak.
Note: This recipe is very thick, not like a normal marinade - more like a paste.