Get them while they're young
Though you have seen a few players retire young recently due to fears of brain damage, many players are saying that now that they know the risks they would still play the game.
And, frankly, it's because they are, for lack of a better word, addicted to it. They've played since they were kids. It's all they know.
The NFL will continue to try to get parents to enroll their children into football. If enrollment continues to slide, the NFL is doomed, and a generation from now it will be as popular as roller derby. If the NFL can convince parents that they are doing what they can to reduce the risks of concussions - either by actual action, or deception - then they will continue to prosper.
But they need the most kids they can to start playing at 8-10 years old, and get them hooked on the game.