The acidic components of most marinades -- such as lemon juice, yogurt, wine, vinegar, pineapple, ginger, or fresh papaya -- break down the proteins in meat, tenderizing it while they impart their flavor. But these acidic ingredients can also react with some metals like aluminum or cast iron and alter the flavor of the food.
Glass is nonreactive, but resealable plastic bags are even better, as they allow the meat to marinate evenly and there's no cleanup afterward.
In a bag, use 1 cup of marinade per pound of meat; in a flat dish or bowl, double the ratio to 2 cups per pound. Don't poke holes in the meat while marinating, since this allows juice to seep out during cooking, and never reuse marinade that has been in contact with raw meat without first boiling it for a few minutes to kill any bacteria.