from wikipedia on sniper doctrines, i guess it helps?
"Snipers are trained to squeeze the trigger straight back with the ball of their finger, to avoid jerking the gun sideways. The most accurate position is prone, with a sandbag supporting the stock, and the stock's cheek-piece against the cheek. In the field, a bipod can be used instead. Sometimes a sling is wrapped around the weak arm (or both) to reduce stock movement. Some doctrines train a sniper to breathe deeply before shooting, then hold their lungs empty while they line up and take their shot. Some go further, teaching their snipers to shoot between heartbeats to minimize barrel motion."