If your shooting in the prone unsupported (holding forestock with free left or right arm and hand) you should use and benifit from a sling. If you are shooting supported, off a bibod, you will not use a sling. Your free hand will be under the butt of the stock. Laying your hand on the top of barrel with free hand is not consistantly repeatable for accuracy at long range and should not be done.
Positional tips for shooting supported (off a bipod) Or at least as best I can remember what I've been taught.
Rifle barrel to be lined up with target; Your spine in line with barrel which is in line with target; Your legs spread apart and toes outward as to have more stable position; Trigger hand not touching side of stock (slapping wood); Triger finger squarely on trigger not canted to right or left side of trigger; Trigger follow through meaning pulling trigger back and holding back through recoil cycle before release; Shoot on total breath exhale with lungs totally deflated; Free hand on bottom or side so butt as to fine tune panning from right to left to line up with target; Squshey shooting bag, such as ones made by Tab Gear, which you position with free hand for increase or decrease in elevation off bottom of rifle butt; Gently pre load bipod foreward with shoulder. Also, keep you eye in the scope.
The main thing with these techniques is to make your built up position consistantly repetable for every shot delivered. Also your built up position should be as much as possible relaxed in nature as it's more easily repeatable.
Also, always use a swivel bipod such as the Harris BRS model or Atlas BT10 if you have to spend the money (Atals bipods are kinda' pricey), and keep the rifle as low as possible with bipod for better stability and repeatable accuracy.
Unsupported is a bit different in positional build up as you are using your free hand to support the forestock and should be using a sling. Also you most likely will not be set up exactly squarely behind your rifle either.
Unsupported prone shooting with sling.