Go easy on the acid in the marinade, which can toughen the proteins in food. Tender meats such as chicken breast will toughen fairly quickly in an overly acidic marinade. For beef, pork, or chicken, use no more than an equal part of acid to oil to make about half cup marinade per pound of meat.
To avoid tough seafood, use a marinade with little or no acidic ingredients (lemon juice, vinegar, and wine are common acidic ingredients). The acid virtually "cooks" the raw seafood. If acid is used, add no more than 2 tablespoons per quarter cup in the marinade. To marinate fish or seafood without using acid, use a non-acidic liquid such as a herb-infused oil, or use a spice rub instead. You can always add an acid for flavor once the fish is cooked.
** Asian Recipes