Gladwell dubs this phenomenon the "10,000-hour rule." I think this can be applied equally to leadership. Becoming truly great at anything -- (leadership included) -- requires ten years of experience and 1,000 hours of practice per year. "Ten thousand hours is the magic number of greatness," he argues.
Becoming a leader requires "deliberate practice."
What are the elements of 'deliberate practice'? It's designed explicitly to improve performance -- the little adjustments that make a big difference. It's repetitive, which means that when it's time to perform for real, you don't feel the pressure. It's informed by continuous feedback; practicing leadership only works if you can see how you're improving.
Bits and pieces paraphrased (and others cut and pasted) from HARVARD BUSINESS REVIEW.