Quick advice: Do daily stretching if your body isn't flexible enough to get a nice, tight position with bone support (losing the gut would be nice, but it takes much longer to see results than daily stretching requires). Once you are flexible enough to assume the best positions possible for you, try "snapping in" - spend some time in position looking at an Appleseed target taped to the wall (scaled for whatever distance you're at). Make sure you have a nice tight sling, and consistent fundamentals (stock weld, sight alignment, proper eye relief, trigger control, breath control, etc.). When in position, close your eyes, take a few deep breaths and relax, then open them. Look where your sights are pointed, and move your body as required to make your sights line up with the center of the target. Rinse and repeat for an hour if possible.
The Appleseed curriculum closely mirrored how I was taught to shoot at the rifle range in the Marines, if you want more details a .pdf of MCRP 3-01A Rifle Marksmanship is probably available from a billion places online and would be a valuable read.