I used to work for a Broadway casting agency years ago. The best thing is to be in as many agent showcases as possible or any off-off (meaning "free" or "Non-union") Broadway shows.
Casting directors attend these or send people from their offices.
One thing to remember: most casting directors will not even look at a resumee that does not have a NY credit on it. It goes straight into the trash.
Therefore, all of your High School, College, or community theatre credits are meaningless. (Except for whatever skills you may have sharpened.)
Attending classes in NYC is a great way of getting involved with other actors and finding out about pending showcases, etc.
"It's not so much do what you like, as it is that you like what you do." SS
"Time flies like an arrow, fruit flies like a banana." GMarx