I'm on my 4th batch of broilers and have butchered at varying ages, and they are always tougher than storebought chickens. I think it has more to do with their living conditions -- the same chickens packed shoulder to shoulder inside a broiler factory seldom stand up and move, so naturally they are more tender. Personally I like knowing my broilers had room to stretch and run in the fresh air, even though it means chewier meat.
I think the ones at the broiler factories are also slaughtered quite young, no older than 7 weeks, which might make a difference.
Resting the birds several days before freezing or thawing slowly in the fridge hasn't made much difference in my results.
Lately I've been experimenting with brining - especially before grilling or using other high-heat methods. A quick google search will lead to tons of brine recipes. Basically, you just soak whole or cut-up chicken in a bowl or zip-loc bag of water mixed with sugar and salt. Does something to the proteins to make the meat more tender. Otherwise, I depend on my pressure cooker to do the job.
I've had good luck with "beer can chicken" on the grill, even without brining. The can of beer (or any other liquid) inside the body cavity helps keep it moist. We used to love baked chicken, but that seems too chewy for my home grown broilers.
