Sticking with one style andand practicing it religeously is the best method for accuracy. Unfortunately for me, I get bored easily and sticking to one style when there are so many different ways to use a sling sounds extra boring I started off with helicopter and got good enough that it wasn't going backwards. After that I've tried to learn every sling style there is and I think I can do all of them now just not all perfectly with certainly not with perfect accuracy. The style I'm most accurate with is Greek but it's also possibly my least favourite style which makes practicing difficult

Same for me I am constantly switching it up and trying to learn all the styles.