First, I'd focus on WHAT you want to do rather than HOW you want to do it; focus on where you want the ball to go rather than your mechanics at that moment. You're mechanics should be automatic, and thinking about them will only mess them up.
To practice this, I'd practice hitting a target that is a variety of distances away. The idea is to absolutely focus on the target but not think about how you are going to get the ball there. Just stare down the target and then throw (do not aim) the ball at the target.
Another alternative is to play a game called "don't make me move the glove" when playing catch with someone. It's exactly like it sounds; play catch with the goal being to not make the person move the glove to catch your throw. Again, the key is to lock in on the target.
The bottom line is that you have to get your conscious brain out of the way and let your body (and unconscious brain) get the job done.
I know this may sound Zen and flaky, but I've found that it works.