Every time you make a decision, your body pays a price. The gravity of each decision is irrelevant, as your brain expends the same amount of energy deciding about breakfast cereal as it does for which home loan to opt for.
To apply this information to your musical practice involves planning out what you are going to do, before you start. By doing this you are circumventing the constant decision making which occurs moment to moment in an un-planned practice session and you will benefit from more stamina and focus in your work.
I created the practice routines (available to annual members) for this very purpose, but I learned about the draining effects of decision making after I had written the routines. These practice routines are popular and useful, but by no means are they the only ones to use. If you invest just a short period of time to constructing a weeks worth of practice, you will be saving all of the energy used to make those “on the fly” decisions and channeling that energy into focused practice.
To compound the effects of this planned practice, you can plan to work on things that actually need work. By this I mean don’t re-hash material that is already working well. Isolate the problem links by recording yourself and spend your precious practice time addressing those specific problems.