The shepherd knew his problem wasn't the slinging: he could hit, instinctively, just about anything he wanted within 80 paces. But ever since he had taken stood in the line and taken the coin-- a whole silver bit for himself-- everything was strange: the sword they had strapped over his shoulder, the orders barked at him and the befuddled others in a strange dialect by the fearsome, scarfaced man in the red cloak, stand, stop, run forward fifty paces, let loose together, shoot ten big stones fast, fall back, lie down-- and over and over, with blows and kicks and curses falling down like hail. 

Now the man in the red cloak had given him a small but surprisingly heavy round shield to grip in his left hand. "Sling !". The shepherd let fly but red cloak would have none of it. "Keep your left hand up !". The next shots were much the same. "Up ! Left hand up !". Finally red cloak stepped out in front, stooping briefly on the way, and stood between the slinger and the target. "Sling !". The shepherd coiled up for his shot and his left arm swung out of the way as it always did but red cloak suddenly dashed up his own right arm and threw and hit the shepherd exactly in the mark of the chest with the earth clod he had picked up. The shepherd staggered backwards, more in shock than in pain, before doubling over.

"You will sling any way you want. But you will do so with your shield up !"

Those bucklers were, of course, for sword fighting-- hence their small size (they protect the sword hand and also trap the opponent's sword).

When you do go snowballing, why not try something simpler ? I mean, drape a woollen blanket over your left arm (like the hides discussed in another thread; in fact, thick cloaks were also used for this). Any snowballs thrown at you, you try to catch in the folds of the blanket. Best would be not to have the stone, I mean snowball, hit your blanket straight on, but at a 45 degree angle, so that the opponent's missile is not only slowed down by the two folds of the blanket, but also deflected towards your left side.

In other words, you sling with your left arm straight, and the left fist stretched out at an angle, about opposite your right shoulder (not with the left elbow bent and the blanket hanging in front of you like a curtain-- you want the blanket to fall at an angle, like a glacis). When you sling, your right arm comes over your left fist (with your right elbow almost coming to rest against your left wrist. 

H... yeah !