Odds of Getting In
Like a lot of things in life—it depends. Do you want to work in one district and one district only? Then it might be a little tougher to break in. Don't really care where you work? Then you'll greatly increase your chances of becoming a principal.
A graduate degree from a solid school, great recommendations from colleagues, and a friendly disposition are a good start. A true leader who knows how to problem solve and who is dedicated to the job will almost certainly find a way in.
Just be patient. You'll have to pay your dues as a teacher first and maybe as an assistant principal before you get promoted to captain.