Structural Stability
OK folks...back to basics from the common shooter...
One thing I learned when I first started shooting from two distinguished marksmen (Bill Dobson USMC and Hank Gableman USAF) was the importance of structural stability. This was reinforced to me again by the great "Ironman" Bill Lee, USMC.
Whether shooting rifle or pistol, relying on muscles to steady the firearm should always be a last resort. Whenever possible, get structural support under the weapon.
"Your skeleton is much more stable than your muscles" I was told, and in the prone position my elbow needed to be directly underneath the forestock of the rifle. Same for the elbow of my shooting hand...tucked in close and in-line with the rifle.
In the sitting position, I always favored an open-leg position with elbows on knees, forming "triangles on top of triangles" to get the best structural support. The sitting position is often overlooked in favor of kneeling, but I've used it while on patrol in high grass, in bunkers, and in buildings (all in Beirut). Sitting provides more structural stability than kneeling.
As for kneeling, I've always turned my trailing foot sideways and sat down on it.
Of course the skeleton is still subject to tremors of the muscles so whenever possible, get something more structural under the weapon.
A backpack is great support, so is a tree. Even better is a sandbag bunker...just be careful not to stick that muzzle out the aperture or window.
Build the habit and discipline of getting the best structural support possible under your rifle or pistol.
Good shooting!