My favorite fajita marinade is something I found off of Cooks Illustrated, which I then tweaked a little (more lime juice and garlic). I've found that it goes really well with both chicken and skirt steak.
Fresh lime juice squeezed from 3 to 4 limes
4 tablespoons vegetable oil
5 medium cloves garlic , minced or pressed through garlic press
1-2 tbsp Worcestershire sauce
1 tbsp brown sugar
1 jalapeño chile, minced (i leave the seeds in but you can remove if you don't like spicy)
2 tbsp minced fresh cilantro leaves
Table salt and ground black pepper
In a bowl, whisk together the oil, garlic, worcestershire, brown sugar, jalapeno, cilantro, salt and pepper. Reserve about 1/4 cup of the marinade in a bowl to the side. Pour remaining marinade over your chicken and/or beef. I've found that pounding out the chicken and beef before pouring the marinade makes the meat more receptive to the marinade, but this is not necessary. I try to let the meat marinate for atleast 2 hours, but if you can do it overnight that's even better.
I grill red and green peppers and a red onion and put a little oil, salt and pepper on them before grilling. After you take the veggies off the grill, cut them up and then pour the 1/4 cup reserved marinade on top and lightly mix together in a bowl.
Grill your meat, put together the fajitas however you like to do it, and you're good to go! I always like a little cheese, sour cream, avocado and cholula hot sauce on mine.....served with spanish rice and refried beans. This might be my all-time favorite dinner to whip together in the warmer months. Hope you like it!!
Pat