The lowest my index was 4.3 when I was a Jr. Golf... fast forward 22 years and playing less than say 3-4 full rounds a year I decided to get back into it.
Background - Swing was very "classic" two plane swing, almost Colin Montgomary like, with alot of timing coming from using my hands and a very aggressive hip turn... Playing about 70+ rounds a year and hitting a couple buckets every day was how I got my handicap so low.
As my body's been plauged/riddled with injuries including mobility in my right elbow (shortening my extention at impact), general flexability issues and a chronic back problem with my S4/S5 and SI joint). Coupled to my old swing and players clubs, I couldn't come close to breaking 90s for the life of me.
NOTE with Adjustments: It takes me close to 4-5 buckets for me to adapt to adjustments and probably playing a couple rounds to not have to "think" about them during the swing.
Hitting the ball Thin or Fat: Most people think that this is all in the hands needing to hitting "down" on the ball... Though hitting down on the ball is the "outcome", most of the time the release with your hands is usually consistent and its all about clearing the hips at the right time.
Because of my back issues it would take me a few holes or maybe like 15-20 balls at the range to not look like a retard. If you were to swing a club at shoulder height, standing straight up with arms ahead of you, you just can see that squaring and releasing the club is usually pretty consistent... What is inconsistent is timing the hips to release the club at the right time. i.e. If you don't release your hips in time, you hit it fat, if you release it too early, you will hit it thin.
My remedy:
To allow for more consistent hip turn, I had to use a narrower stance. I just couldn't use a shoulder+ width stance that most people use to keep their lower body still. As my head/hips are pretty used to being quite still throughout a swing, using the narrower stance allows me to turn properly with less "pain".
Timing and wight shift: With my "joints" now better aligned to go through the swing motion, timing needed to be re-learned to ensure the hips are re-centered at the begining of the down swing, then shifted open with weight on my left side during the swing, clearing the path for the club face to be centered at impact for longer and ensuring power goes through the ball utilizing the correct loft and shaft load/lean.
My remedy: Think about your "right hand" in my case (right handed swing) and skipping rocks... Lead with the hips (lower body), arms and keeping the lag in the hands through to release. Just like skipping a stone, you will keep this action on the same plane and groove (finishing with the same body lean) such that you can easily add speed to cast the stone farther. Since doing so, I've not had to think nearly as much or at all about my takeaway and hitting it less fat/thin.
More solid contact and less motion: Once in a while when I feel off, I lose the ball on the tow, high and to the right. Why? I was used to a two plane "classic" swing being armsy and I was relying on my left hand too much on lag and release through impact.
Remedy: Stupid but simple, on a single plane swing you want your backswing to leverage the body more than the arms. For me making sure I keep my left arm is glued as tight to the chest as possible, providing that consistent "guide' to ensure that I bring the club back to address consistently.
The other is to focus on the right hand and keeping the grip as a single unit. Just like casting the stone into the water, I think of casting that club head using my right hand into the ball, into the ground. Since doing so, I've been much more consitent with my divot depth and catching the ball more solidly. Even if I lose it on the toe, Iat have had a far better chance of directing and compressing power through the ball.
Typical foundations are always in play... keep everything as still as you can, don't lift or turn your head until after impact, and keep your body lean as long as you can (if you can)
in short my tips for more consistent ball striking:
1. Fat/thin - Use a stance that lets you clear your hips consistently, and think of timing as if you are trying to skip a stone
2. More consistent and solid contact: Use a one plane swing and keep that left arm squeezed as tightly to the chest as possible and as "guide". Using that same skipping stone mentality think of using that right hand to deliver the power... holding the lag and releasing that club face using that hand into the ball -> ground
3. Adding Power - yes its all about speed, but rather than thinking of speeding or over-extending your swing, I think about how hard I drive my right heel into the ground during the backswing. Most people think about over extending the backswing, but when really its about coiling the resistence against your driving leg.
Using these tips and coupled to my short game, in just a few rounds I've managed to shoot low 80s again, which is more than "ok" for me for someone who just plays recreationally.
I've also switched to game improvement clubs, and though they help on off center shots closer to the toe, there are two things I "HATE" about them: 1. Loss of Distance on good shots... yes though you can swing for the fences with less risk, on-centered shots are balloney and shorter. 2. Can't work them. Trying to hit anything other than a moderate fade or draw, you'll just fuck up as the fatter sole will catch. Its like trying to hit a finishing nail with a sledge hammer in a small space... good luck with setting up a "stinger" with an open stance and the ball closer to the back foot. 3. Divots... especially when the ground is hard, the fat sole and bounce discourages a steep angle which I prefer on short irons for pitches or when trying to hit a spinning high fade.