I use a different approach to training than what most books will tell you. I train to exhaustion of the muscle or some days it is my mind set. Most days for the muscle group I am working I work them until I can no longer lift a reasonable amount of weight. Some days might be heavy days and after 8-10 sets my muscles are just exhausted, other days I just feel the energy and I might be 20+ sets deep dreaming up new isolation exercises and 2 hours in the gym later. I always start with a heavy compound exercise like deadlifts, squats, bench press, standing overhead press, with arm day being my only exception as arms are all about isolation and it is a day to let the rest of the body recover.
Now comes the mind part. If you hit the gym and your mind isn't in the right place, you might do 5 sets and if you can't reach your head space the workout is over. If you try lifting heavy and your mind is not in the right place you are increasing your chances of injury. I'm 47 years old going on 48, would rather have a couple of short days in the gym then a couple of months nursing an injured back or joint. We all have lives outside of the gym and sometimes your mind needs you to take care of those things before you can hit the right head space.