In a belt pouch, on my weak side opposite the gun (typically about the 8:00 position), upright, but with the bullets facing forward. If the bullets face to the rear you will either have to turn your hand 180 degrees to orient the mag properly, or turn the mag in your hand to insert it into the gun. This is not only a wasted motion, but could be fumbled under stress. Your index finger on the front side of the mag helps to guide it into the well. I also dislike carry of dual mags where they face in opposite directions. Also, with a pistol, if you carry the mag on the same side as the gun, you have to reach across your body to retrieve it. Revolver spare ammo can be on the same side.