2 Answers

1.  Select your chicken carefully. In the U.S., most chicken sold is young, but getting an old bird isn't an impossibility. Know your source, as older chickens yield tougher, stringier meat.

2  Consider the cut of poultry used. Dark meat chicken contains more fat, and therefore can be more moist and tender in dishes that work well with dark meat.

3  Do not overcook the bird! Overcooking can lead to loss of moisture and proteins tightening up into bubblegum consistency. Experience will help you judge when chicken is done, but shoot for the point when the meat is just opaque in the thickest part of the cut. Use a probe thermometer when cooking a whole bird, plunged into the deepest part of the breast. Use an instant read for smaller cuts like breasts and thighs.

4   Use a "low & slow" cooking method. Though seemingly contradictory to the "do not overcook" warning, low, slow cooking breaks down proteins, allowing for a more tender experience. This type of cooking also allows time for flavors and juices to penetrate into the meat, giving even more juiciness.

5   Marinate or brine before cooking. Marinating smaller cuts with flavorful liquids for at least an hour, preferably several hours before cooking, or brining a whole chicken overnight will infuse liquids and flavors into the meat.

6   Pound boneless cuts, such as breasts, before cooking them with a fast method, such as grilling or frying. Pounding both breaks down fibers in the meat and allows for very fast cooking.

7   Allow the meat to rest after cooking. Cover with heavy-duty aluminum foil and let sit for 5 to 10 minutes. This allows juices to redistribute in the meat that would otherwise run out if cut immediately after cooking.

8   Mulch a Kiwi fruit without skin (or Paw Paw/Papaya, mango or lemon/lime) and mix with cut chicken (or beef) to tenderise covered in the refrigerator for about an hour (no longer). This works best when the kiwi fruit and chicken pieces are also mixed with a paste of curry powder and water. Alternately, use curry pastes like massaman, madras, panang, etc. You cannot really detect the paw paw or kiwi fruit flavours in the finished curries, but you may in other dishes - use common sense.

 Tips

All other things equal, fresh chicken will be more tender and moist than frozen. Freezing breaks down cells in the meat, causing loss of moisture, which can mean a tougher bite later.
Commercial meat tenderizers may be tempting, but are unnecessary chemical additives if other steps are followed properly.
Pounding chicken is often done with a mallet between two sheets of plastic wrap or waxed paper. However, zip top bags (with air pressed out) work best, and any heavy object will do, for example, a rolling pin would work fine.
The FDA recommends that chicken be cooked to 165 degrees Fahrenheit for safety. However, when roasting a whole chicken, remove it from the oven when your properly placed thermometer registers 155 degrees. Allowing the meat to sit under foil after removing will allow "carryover heat" to raise the internal temperature of the bird above the danger zone.

according to my late husband, who was a great Italian/American  cook, He put them in Italian dressing overnight, and it was a great marinade. Made them tender and tasty. Then, he'd grill them outside, even in the winter. Personally, I like to put them in Teriaki sauce before cooking or grilling.