First, make the rifle as tight in your shoulder as possible. If this is painful, you are doing it right. Try positioning your body slightly perpendicular to the target and cock you leg(bend knee) on shooting side may help. Third, make sure you have a natural point of aim, by adjusting your body so that the sites are on target even when you look away from the sites. Use a sling around your arm to make the rifle butt fit tight into your shoulder.
With enough practice your body will loosen up and be more accomadating.