the proper way (read: the way i was taught) is to keep your elbow in. minimizes your target and keeps your profile as small as possible.
remember the butt of the gun is not technically on your shoulder, but rather on the space between your shoulder and the edge of your pectoral. i think its call the nook of the shoulder or something, i dunno.
Either way, elbow in and when sighting down your gun, dont bring your head to your gun, but rather your gun to your head and DONT crane your neck to the side, but rather keep it straight.
Bring the gun up to your eye level and align the scope and gun with your eye.
hope that helps.
(for all your Gun Nazis, if i got something wrong, please correct me
)