Corn starch! I used to think putting corn starch in the marinade just helped to thicken the sauce when you stir fry, but the chef, Ming (from the show Simply Ming) said it tenderizes so I tried it on chicken and it came out with that same consistency and tenderness you get at the restaurant! Tried it with beef and it also worked, but admittedly not quite as well as it did with chicken.
Use soy sauce, fresh grated ginger, corn starch, and a little water to make it more liquid, and marinate for 4-24 hours. Stir every few hours so corn starch doesn't stay sunk to the bottom.
