We went to our BIL/SIL's house last weekend for dinner and they BBQ'd skirt streaks they had marinated - it was amazing. I seriously could eat it every day and I am not a huge meat eater. I should add that they are master BBQ-ers which might have helped.
Adapted from Alton Brown's recipe.
1/2 cup olive oil
1/2 cup soy sauce
1/4 cup dark brown sugar
2 or 3 cloves garlic
1 bunch (4-5) scallions
1/4 cup lime juice
1/4 teaspoon red pepper flakes
1/4 teaspoon onion powder
1/2 teaspoon ground cumin
Throw all that crap into the food processor and hit GO until you have a smooth mixture. Empty into a large ziplock. Add skirt steaks. Refrigerate. Wait patiently for a few hours. Take them out of the fridge and let them warm up a bit before grilling on an absurdly hot grill. Lot of sugars in this marinade, so be careful you don't cross from "caramelized" to "burnt".
Re: You MUST try this marinade for steaks: