Marinades are used for tenderizing as well as for adding flavor to meats.
The acid in marinades like vinegar or citrus juice work by breaking down connective tissues. Since the reaction between the meat or veggies and the marinade is a chemical process, there are a few factors that can affect a recipe. Time is a key factor, of course, but temperature is crucial. The colder the marinating food is kept the slower the reaction will be. The longer a food marinates the more tender it will be but too much time may impart more flavor than you want.
The amount of marinade is important as well. As mentioned elsewhere zipper bags are indispensable in the kitchen and this is a perfect example because you can use less marinade and still get maximum coverage. Plus you can "turn" the meat in the bag easily to coat it completely.