This isn't rocket science, but the two keys are adaptability and recruiting of good players who fit the system, and not necessarily in that order.
All the talk about Saban's adaptability is true and very much to his credit, but it's also made a hell of a lot easier with 4 and 5-star players at the skill positions and across the OL, and the ability to coach them up. At this level, any system will only be as good as the quality of players recruited and the how well the coaching staff can grow the talent, and no one has ever been better at it than Saban.