My guess is most of us don't have a set baseline for mental performance, and being able to establish one would help move us all forward. Yet, especially when it comes to the brain, how does one set a baseline? Sure, there are some basic tests like word and number-spans, mental object rotation, etc, and I do want to improve on those tests - but what about complicated mental creativity? If I improve my dual-n-back, will my writing skill and accuracy really improve? Entirely plausible - but difficult to say. Dual-N-Back is just one among many competing baselines.
And of course, once you have chosen a baseline for mental activity, how do you create isolated experiments? You may sleep less or more at night, but how do you know that's having the effect, and not the time you woke up, or the timing of your breakfast?
A possible answer: Results must be big.
Sorry about the vague, philosophical post, but I'm feeling vague and philosophical at the moment. And unsolved.