Brine or marinade first and don't over cook it. That's it. Poultry is ready at 180-degrees. remove it from the oven or pan at about 170 (it will continue to cook as it rests) and let it rest for 15 minutes before cutting into it.
One thing I find is people wanting to eat "healthy" so they buy boneless skinless chicken, that's the driest way to have chicken. If that's what you're using try using chicken that has not been stripped of it's natural marinade (skin) and flavor enhancer (bone).