Blindfold slinging.
If you are going to train for a fixed target at a fixed distance and always use the same sling and ammo and style.
Then the ultimate goal is to totally eliminate thought.
It's the exact same motion.
You are training muscle memory.
So, eventually you should be able to do it blindfold and just let your ingrained muscle memory take over.
I always find with target throwing that my throws during practice - before I start to think about it, are generally the best throws of any session.
So for competitions, I generally don't bother doing the queuing up and practicing a few shots any more.
And my first round Is frequently the best.
Unfortunately I don't do enough practice target slinging to keep the same mindset throughout a full competition.
At some point I'll think: 'what if I did this, or that'
At which point everything goes all to hell
But it is as simple as that.
Same thing over and over and over until you have eliminated thought.
At that point you should be able to do it blindfold
On a side note, there's a video of a blind woman learning to sling in Guam, that's worth looking up.