From my experiences most people that turn it early are trying to hook the ball more than they should for the positions their body is in.  Not enough leverage.  There is probably something either in your backswing or transition that is causing this.  You need to video yourself.

The first two suggestions can be quite helpful, and I'll add a third. I think that part of it is a mindset, and it's something that I'm working on as well. I have always been more of a "thrower" of the ball when it comes to release, and I've always marveled at the men and women who are "rollers" of the ball.

You need to think of rolling the ball onto the lane at release instead of throwing it onto the lane. I think that Chris Barnes provides an excellent example of this. Even when he's playing inside, you can still see his hand go straight up the back of the ball at release. That's the kind of feel you need to be shooting for.

You can drill this by going up to the foul line, taking a knee, (kneel on the leg of your push foot, and keep your slide foot planted) and just practicing letting your hand come up behind the ball with your fingers coming straight up the back and exiting the ball last.

Once you get the feel of that, which could take a while if it's something you aren't used to, move on to trying it with an approach. What I found is that it really wasn't hard to come straight up the back of the ball. The issue I had was finding a way to do that without causing the ball to just tumble end over end with a 12-6 roll pattern. After some experimentation, I found that I need to change my setup in order to impart at least some axis rotation and tilt. However, I also found that rolling the ball 12-6 is fantastic for spare shooting. If I'm disciplined in my release, I can throw my most aggressive balls at spares if I want to because the lack of axis rotation keeps the ball on line.

Long story short, it's just going to take some time and practice like anything else. Try it out, give it some time, and don't expect dramatic results right away. If you can do that, you'll likely be rolling the ball better in no time!

As an old school stroker, my best way for staying behind the ball is to keep my arm very relaxed, especially through the elbow, and the grip.  When we grab or hit up, the slightest timing problem will cause you to come around the ball too much. 

I spent the whole night in league last night fighting this same problem, and it wasn't pretty, except for pretty frustrating. Just reviewed Ron Clifton's video and will try it, but I also stumbled on Norm Duke's "Early Turn" video. Loved his presentation style and the tip as well. The man's my hero now in a new way.