Interesting topic Old Fella. Some of the comments on here assume "full, complete, effort" already. For myself, I've found that I probably have never put forth actual "complete" effort, therefore some basic measures worked for me.
My shoulders were weak. The fact is they were always a source of pain on average nine months out of the year and often made a neck problem flair up. The result: "For years I would "short change" shoulders". If my workout cycle involved missing a day that week, it would be shoulders hands down. The next week I would pick up at the beginning rather than start with shoulders. That is one example. If I was on vacation traveling and got some gym time, you bet your hiney it would not be shoulders. Now please understand that this was not a conscious decision. It only seems clear looking back on it. (The is the best vision isn't it).
When I focused on shoulders, put time into a program design and adopted a "shoulder" first attitude, results followed. If I missed a few days and had to restart my training sequence, it was shoulders, etc. In one year it made a huge difference. So for me, it was basically mental approach. I was practicing a form of the "beach muscle" boys syndrom. I would do what I was best at, and ignore what hurt.
I recently shifted my focus to back and traps. Please understand I have been fearful of trap, neck, work for years because of getting "spasms" that were debilitating. Guess what? The extra back and trap focus improved my "balance" and neck spasms are a rare occurence and I have made gains.
At my level, non elite, heck, non-average even, I have so much head room left to make gains that the solutions like Syntherol are totally unnecessary. I am going to bet that for the all but the elite, this is the case.
I always thought I was training with a full, balanced, effort. That is the important point. It is difficult to admit to yourself that regardless of the relentlless balls to the wall workout warrior you invision yourself to be, you are still leaving something on the table.
Its kind of like that example of filling a jar with marbles, thinking its full, then with BBs, then with Sand and then with water. Each time it was "full" it had plenty of room for more. We are typically not as "full" as we think we are!
I wish all things in life gave a true a payback as focus and effort in training!