accuracy actually sneaks up on you.
During the hillfort experimants I discovered that I'm extremely accurate. And can now state that - yes I could defend myself with a sling and hunt with one too should the need arise.
Best tips I can give are simple:
Always use the same sling. I think that's the most important thing.
I've carried my cap paul for about 6 years. rarely use anything else and when I tried target shooting with another sling I was rubbish.
But the cap paul is now just like an extension of my body. Whatever style I use it feels natural.
And once you stop thinking about the basic mechanics of slinging you can start concentrating on the finer aspects needed for accuracy.
I would also recommend spending 10 years slinging stones, balls, bottles, sticks, etc for a dog.
In particular a running dog. Turns out that's the secret to accuracy.
When you're slinging 1lb rocks to land fairly close to your best friend - you get accurate
Consitency is also the way to distance. The more you sling the more you'll start using your whole body and the better your power slinging should get.
In other words: practice, practice and then when you get bored - practice some more