I went out this morning to get more practice in with the collection of shot I rounded up.  
 
I started out with a helecopter style but every throw was to the left (left handed btw) regardless of how I changed my stance or angle. I swiched to the underhand with a bit more success but still not hitting the mark (large round of hay about 25 yards out.)
 
I eventually went to the overhand throw and instead of hanging the pouch behind me, I held it out toward the target. With this my accuracy went up greatly. I actually hit the target a number of times and very few wild shots.
 
It felt like being in an Easter egg hunt when I went out to retrieve what shot I could find.

  I proudly proclaim that I can hit a man in a crowd.
 
Not sure how well this phones video capability is but I may put the wife up to testing it.

This may just mean that helicopter is not the style for you.  
 
I really like it but it took awhile to really be good at it. You may also be using heavier ammo than normal.

Got video just need to upload it. Unfortunately my wife isn't real great at using a camera.

I dug up my long distance tape and got a more accurate measure of distance tothe hay roll I have been using as a target. I had been throwing at about 20 yards. I marked off 25, 30, and 35 yards (7.62, 9.14 & 10.67 meters).
 
Since I was at the 35 yard mark and armed, I had a go at the target. I hit it far more often or if I over/under shot I was in line with the target than I was doing at 20 and 25 yards.

Some time ago I have in detail an exact one shoot described.
 
They say which her hits lay often too far on the left?
 
Often lies with too much strength in the shot.
It needs some time till one full power controlled.
 
My tip: if they shorten her Sling.
If they leave out the aim.
If they practice just from to shoot.

Jaegor, it varies person to person. If you use one almost consiststent set of stones, and then switch to a larger/heavier set of stones, your aim will be off and will need to be adjusted.
 
CA, I can mostly agree with you about Heli style. 100% helicopter is good for looks, but is more difficult to use than many others. Modified helicopter used by several like myself and Yurek use can be very effective. Yurek's videos speak for themselves and with practice I find it to be very powerfull and my most accurate style (I can usually hit a baleraic size target pretty much every time up to 20 yards or so with good rocks). How to modify it is up to the person using it, I start with a greek grip and make my throw a little more verticle like underhand.
 
I don't have a video of my personal slinging style yet, but I will soon now that schools out.

Ok, getting the videos uploaded.  
 
This one my wife did the majority of it sideways
 

 

 
This last one was with baseballs but my wife wasn't back far enough to catch the flight. I didn't hit the target either time tho.