Something that has helped me lately to get the club at the correct place at the top is Nick Faldo's YouTube tip/drill on the take away. I am on my phone so don't have link, but is easy to find. A DRILL he recommends is standing at address, turning hands 90 degrees so club is parallel to ground and parallel to target line, then just turn your shoulders. Now you are positioned perfectly at top of backswing. When I am worried my backswing is getting off plane, I might do...
Assuming your grip isn't too strong, focus on a good hip turn. Hard to come over top with good hip turn. Downswing is started with turning hips, then shoulders, arms, hands in that order. As long as I make good hip turn, neutral grip, and swing down target line, then I keep from hooking.
My local course doesnt have a driving range (no range within 40 mins), so I have been taking advantage of a local soccer field to practice my short irons. Really seeing improvement in all aspects of golf game (except putting- my new focus).
After studying and working CORRECTLY on my game this summer, I think I am about to drop into single digit handicap for 1st time of my life. Answering your question in %'s as you originally asked I'd say, to improve at golf:
50%= learning the correct fundamentals (for starters study the book Hogan's 5 fundamentals... It's the bible. I attribute most of my recent improvement to it.)
30%= commitment to practicing these fundamentals and working hard to improve in general....
I think my handicap is about to drop into single digits for the first time in my life due to this book. I've worked hard to copy everything. Only deviation is stance (foot position always parallel to target line). Whenever I start missing I eventually always realize I deviated from his swing instruction in some way (off plane backswing, weight not slightly more on heels, left elbow not pointing at left hipbone, etc).
Just need to fix my putting and then look out.