Just keep practicing. There isn't really any clear cut way for everyone to find the perfect release points for every pitch.
One thing I've been told in the past, even in games, is to slow down my fastball in search of more accuracy. I don't really agree with this, because then I'm instilling the muscle memory of throwing like 50-60%. Plus when I take that much off it totally kills the movement that my fastball has.
On top of that, who says you want the same release point anyway? You would never want to throw the pitch in the same place over and over. I'd just practice to be able to throw accurately, so it's not even a thought as to where you release- it just happens.