Definitely the fit is a big one. I shoot my Parker-Hale converted Lee-Enfield No1 the best out of any of my Lee-Enfields because it has a much better length of pull and higher comb plus a decent sling and a nice balance. It feels rock solid in my shoulder compared to all my other rifles.
I read the old military musketry manual 'Shoot to Live' and practiced it a lot when I first started shooting. I got good enough that I could squeeze off a dry shot with an empty .22 case balanced on the front sight ear as well as my position figured out enough that my 120lb wife could sit directly on top of my rifle at my forward hand indefinitely (prone obviously). I took what I learned there and moved it into offhand, elbow directly under the magazine, relaxed forward hand grip, tight cheek weld that supports all of my heads weight, rifle moved to my eyes and not eyes moved to my sights, proud right elbow, foot stance and weight on the forward foot.
My Dad also taught me to practice offhand with my heaviest rifle, or one that is balanced far forward (SVT-40 and Garand work well).
Lastly, shoot lots. Even a .22 will help you tons.