I usually start 1 day ahead for beef. The night before, I sprinkle both sides of the meat with Adolph's meat tenderizer and put back in the fridge tightly wrapped overnight.
In the morning, I marinate in 2 cans pineapple juice, about 1/2 c light soy sauce, Juice from 2 limes, 1 coarsly chopped jalepeno, and a couple of crushed cloves of garlic. I usually don't marinate overnight unless I'm having the fajitas for lunch so that I still get a good beef flavor, however, if you want to marinate overnight go for it.