Since so much advice has been listed on this discussion, I might aswell add what I find to be most help full. I allways let my sling dangle for a few moments to insure the strings don't have any twist, and adjust as necessary.  Then I make it fly.  I find just doing what my body wants to do increases my power and accuracy noticably.  My margin of error is very consistent even if the stones and posture are different shot to shot.  For me, slinging is the kind of thing where the most I can do is make sure all the preconditions are right, but the act itself is governed by instinct alone.   

I dont think these "sail away" shots are a problem.  They are moments when everything went right.  I want to grow into them, not wall them away with precision and discipline.  It's said we are born knowing how to swim...