Marinades add flavor and tenderize tough cuts of meat, such as the skirt steak used for fajitas. To help tenderize, true marinades include one acidic ingredient such as lemon juice, wine, vinegar or enzyme-rich fruits like papaya. My favorite quick and easy marinade for fajitas combines Newman's Own Olive Oil and Vinegar dressing with soy sauce, fresh onion slices, freshly squeezed lime juice, and some sprinkles of fajita seasoning. Toss everything in a large Zip-Lock plastic bag with the meat and lime peelings, then marinate in the refrigerator for several hours before grilling. To protect against food poisoning, a marinade that has been in contact with any kind of uncooked meat must be cooked to a full rolling boil before it can be used as a sauce.
Rubs add flavor and can form a crispy crust on the food.
Since rubs lack the oil usually present in marinades, rubs add flavor without adding fat. For a rub with a slightly-sweet flavor, try the winning seasoning mix from the Texas Beef Council's annual Signature Steak Rub Contest. See their brown sugar-based recipe on this page. Or try an unusual twist on a rub with Beer-Can Chicken. It might look funny, but this method can bring tasty results.