What makes chicken tender -- or tough?
Don't let chicken dry out in the refrigerator; dry chicken is tough chicken. Keep it wrapped in the package it comes in until you use it.
Avoid freezing it. When the juices inside the cells freeze, they act like little spears and they'll rupture some of the cell walls. When you defrost the chicken, you'll lose some of the juice and the chicken will be less tender.
Cook chicken to the proper temperature, using a meat thermometer or pop-up guide. Cook bone-in chicken to 180 degrees and boneless chicken to 170 degrees. Undercooked chicken will be tough and rubbery because it takes a fairly high internal temperature to soften the proteins in the muscles and make them tender. But don't overcook chicken either, because moisture will start to steam off, and the more chicken dries out, the tougher it gets.
Keep the skin on chicken during cooking. The skin helps keep juices in, and tenderness and juiciness go hand in hand. I've tried this both ways, and the difference is significant. (When you cook chicken with the skin on, approximately half the fat from the skin is absorbed into the meat; if calories and cholesterol are very important to you, you might want to remove the skin before cooking even if it means a less tender result.)
When microwaving any chicken product, cover with a loose tent of waxed paper to prevent drying.
Some authorities feel strongly that you should not salt the chicken before cooking because salt draws the juices out during cooking and toughens the meat. In my experience, there is a detectable difference in tenderness between salting before cooking and salting afterwards; the chicken that I salted afterwards was slightly more tender.
Still, I would guess that most people, myself included, wouldn't notice a big difference unless they were specifically paying attention to it. The difference doesn't jump out at you as it does with overcooking or freezer burn.
Fry or roast breast pieces rather than microwaving them if tenderness is a top priority for you. Microwaving is significantly faster, but there's a greater risk of toughness when you microwave breast meat. Breast meat is fairly dry to begin with, and you don't have a whole lot of latitude between overcooking and undercooking. With breast meat, there's a trade-off between the speed of microwaving and the reliability of frying or roasting.
From the Perdue Chicken Cookbook