You can strengthen yourself in the weight room - lateral raises and military presses work the shotgun-lifting muscles - but the easiest way is simply to practice gun mounts. Start with the gun in the unmounted position shown in the picture. Raise it smoothly to your face (always bring it to your face first, then settle it onto your shoulder), concentrating on form, not speed. Repeat. When you first start, you'll feel the burn after about 15-20 repetitions. Work your way up to 50 or 100. Not only will you build endurance strength with this exercise, you'll develop the muscle memory you need to mount the gun correctly without thinking when birds flush unexpectedly.