Properly training for anything is about reducing the variables. The broader the skill set a coach has the more variables he can reduce for his players and clients. In most performing situations, the amount of variables you can potentially eliminate is endless.
The first step is creating a hierarchy of importance. Skill specific items are usually top of the list. For example, in golf you can create a very repeatable face to path relationship and consistent low point. That alone could knock down your handicap 10 points. The further and further most athletes and performers get into the skill specific variables the more they start to look for non skill variables to be eliminated such as energy levels and focus.
A golfer may put a lot of time practicing reading green slopes and grains, but may realize that if he has an abundance of mental energy his database of green reading is much more accessible.
Let's look at a highly simplified example of the above situation.
Golfer 1 has put in 100 hours of green reading practice, and his sleep, nutrition, and daily habits are such that his memory and processing speed allows him to access 90% of that practice on a given day. So he's effectively putting with 90 hours of experience, and each hour he practices he adds 54 minutes of net productivity to his database.
Golfer 2 has put in 500 hours of green reading practice but his daily habits to fluctuate his access from 40-60% of his database. He's putting with 200-300 hours of experience on any given day. One hour of practice for this golfer will only yield about 30 minutes of net productivity.
This is an obvious case in which Golfer 2 needs to practice better lifestyle habits and eliminate the variable of his mental focus being an issue, and Golfer 1 just needs to keep practicing green reading and putting. One thing to keep in mind though, a severely hungover Golfer 2 would still beat a fresh Golfer 1.