I tried this Balsamic Garlic beef marinade, and it was wonderful! Best part is that you don't even have to remember to do it ahead of time--a short marinating time and then grill or oven for a short time and you have some great flavor added to your beef while still tasting the beef.
I used close to her amounts of the ingredients. The original recipe comes from Food and Wine.
Balsamic Garlic Beef Marinade
3 Tbsp balsamic vinegar
3 Tbsp olive oil
2 cloves garlic, crushed
1/2 tsp thyme leaves
1.5 lb skirt steak (she used flank, but it also gave great flavor to skirt)
salt and pepper to taste
Combine all ingredients (suggested to use a blender, but I just minced my garlic and whisked it together). Pour over meat and marinate for 5-10 minutes. Cook meat, allow 5-10 minutes rest time, and serve!