I am by no means a lights-out shooter, but I've improved exponentially after one big change: I got a new shotgun.
Now hear me out, because I know what you're thinking, "right, buying a new gun is going to fix everything. Sure it will." Actually no, that's not why I improved. It's because I finally got a gun that FIT me, and I didn't even realize it until after I bought the gun.
So I hunted for many, many years with a run-of-the-mill 870. It went bang every time, but I missed WAY more than I hit. I was the butt of all shooting jokes among the friends I went with. And the more I missed, the more I got ribbed...which stressed me out and made me miss even more. I overthought my shots. Would pull off a miracle wonder shot and then proceed to miss every softball that came at me. It was brutal.
This went on for the better part of a decade. No joke. I practiced constantly but never improved.
I finally started reading about shooting and kept stumbling across the same advice: Get your gun fitted. Most of this talk applies to high-end two-barrels, where stock fitting is just part of the gigantic bill. But when it came to factory-made guns, there's not many options. I eventually started playing with stock adaptors that raised the comb and adjusted the length of pull (LOP). I suddenly started hitting better. Not huge, but enough where I realized this was probably the key to my shooting woes.
Finally I decided it was time for an upgrade to a semi-auto. I scratched and saved before finally buying a Benelli M2. As with all Benellis, it came with stock adjusting shims. So the first thing I did after cleaning the gun was play around with the shims until I found what fit best.
I can't say for certain this is what has improved my shooting so much, but I'm way, way better of a wing shooter now in the past few years of having the Benelli than I ever was with the Remington. On Sunday morning I hunted ducks solo and bagged my six in 11 shots. (That includes shooting three times at a cripple, which eventually got away). Then after a quick walk for pheasants, I bagged the lone rooster my GWPs put up in a single shot. Seven birds in 12 shots. In years past, that would have been the best day of my season, but since getting a gun that fits me, it's on the good side of average.
I'm not trying to brag, just pointing out what helped me. And I'm by no means saying you need to go out and get a Benelli, but seriously analyze your gun and how it fits. I found with my 870 that when I anchored the gun, I wasn't seeing the front beed. By getting a comb riser, I managed to make it feet me much better.
After the fit, it comes down to shooting form. It's all muscle memory after awhile, but being sure to always bring your cheek down to the stock and having a solid anchor will greatly improve your shot-to-hit ratio. I found with my 870 that I was lifting my cheek off the stock to shoot. Sometimes this worked, but most of the time I was horribly inconsistent. Plus, when I did hit something, it was usually crippled.
Do yourself a favor and go to a gunsmith. One worth his salt can help with gun fitting. Or at the very list, shoulder your gun a few times and see how it feels. I used to put a small Mag light in the barrel (unloaded, obviously), find a point in the room, close my eyes, shoulder the gun and see if the light was shining where I wanted to point it. That was a big indicator that the gun didn't fit me, because the light was almost always lower.
Anyway, that's my long-winded 2 cents. Good luck!