Odds of Getting In
Both Esa-Pekka Salonen and Gustavo Dudamel snuck in by filling in for a conductor who fell sick at the last minute. You'll likely increase your odds if you're around people in powerful positions and show some promise.
There are a finite number of spots out there and many of them are already taken by famous conductors. You'll have to work your way up as someone's apprentice and hopefully make the right kinds of connections that will get your foot in the door. It's not going to be easy.