I am no professional when it comes to ballhawking. In fact, there are studies out there that you can look up. But since I am terrible with math, I will tell you about how I do it.
In the last five games in which I attended the game early for batting practice, I have gotten four balls. None of these balls have come by catching it. But catching a ball is still the ideal way to get a ball. Positioning yourself in a spot in the outfield where you can easily move around is the best bet.
But other than that, here are some tips about how to ballhawk (from my experience):