Have you ever thought why do we marinate meat?
It is for flavor they say, but how does that flavor get in? The acid in marination tenderize the meat they say, but how does that happen? What are the loopholes in this process?
Apparently, acidic solution starts the cooking process in meat or fish. Acid denature the meat (meaning loosen the muscle structure of the meat and opening up tunnels), so that the flavor part of the marination can penetrate the protein. However if you keep it in the marination for a longer time, the acid starts the cooking process. Since chickens (chicken breast predominantly) and fish have a less dense muscle structure, they are more prone to dry out if kept in a marination (acidic) for longer time.
Then I found another way of keep the meat juicy. It is call Brining. One of my culinary gurus Heston Blumenthal, explained it in one of his cooking show, “How to Cook Like Heston“. It is easy, just take some water and add some salt, the magic number is 60 gms per liter. Then immerse the meat in it, preferably overnight. And what do know! It works like charm. Every time, we get tender meat.
 Check out from 3:20 mins to 3:50, Brining Explained by Heston Blumenthal