The Pareto Principle (also known as the 80/20 rule) argues that 80% of the rewards come from 20% of the efforts.
Think about this for a moment in terms of your music practice. This means that 80% of your time could effectively be wasted. Awareness of this fact is the first part of the cure for practising musicians.
For this practice method you are going to ensure that not only do you use all of your time as well as you can, but you also use the most productive time to work at the weakest things.
Firstly you are going to split your weaknesses down into 5 easy chunks. Have a think about what the 5 weakest areas of your playing are. Is it scales? Tone? Fingering? Reading? Embouchure? Write these 5 things down in order of weakness.
You should now have two lists, one for weaknesses (list A) and one for tasks (list B). Each is split neatly into five 20% chunks.
The next thing to do is to tackle them in the right order. You want to spend the most productive part of your practice time (the start) working on the most important items. Therefore start with weakness 1 (A1) and then move onto task 1 (B1). Next would be weakness 2 (A2) and task 2 (B2) and so on down your list.
Using this practice technique the Pareto Principle might still apply. However, you will have stacked the odds in your favour. The 20% (the start of your practice) affects the most important things you need to work on.
Attached is a music practice sheet for download which you can use as a template to create your lists.