How do you make this poll and not include "Practice"?
That's the secret to every skill known to mankind. Practicing. And not just mindlessly - practicing perfectly. Efficient and accurate practice is the #1 (and only) route to improving at everything under the sun. And really it sums up everything already mentioned. Rules knowledge, flipper skills, aim, decision-making under pressure, ability to adapt to subtle differences in the same machine at a different location. All stems from practice.