Some general principles that applies to training:
* Frequent training reduces the total number of hours required to reach a given standard compared to training only infrequently. A couple of times per day is better than once per day which, in turn, is better than a couple of times per week, in turn better than a couple of times per month, etc etc. This is most critical during the initial stages of any skill development.
It's always struck me as backwards that many (most?) people start by flying once or twice per month & then, as the flying bug bites, fly more & more often towards the end of their training. Far better if the initial part of their course has frequent flying & then, if absolutely necessary, tails off towards the end.
* Younger student is better than older. Reaction times are reduced & rate of learning is faster. On the other hand, my experience has been that many older students have maturity & discipline to ensure that pre-lesson study is done and also a lifetime of experiences that can be applied to the new task (on the third hand, some older students are doubtful about the skills of a much younger instructor which can hinder their rate of learning. Not really the instructor's fault though...)
* ***Good*** weather ie smooth conditions & excellent visibility with a clearly discernible horizon is important for the student's maximum rate of learning during the early stages ie first flight (effects of controls) to first solo & consolidation (early morning lessons, anyone?

). Thereafter increasing x-winds & turbulence is beneficial.
* Learn the 'theory' on the ground, not in the air. As was mentioned earlier in this thread, aircraft are **terrible** class rooms. They're not the place to discover the how's & why's of doing things. It's cheaper to learn 'how' & 'why' in your own time than paying $$$ per minute in the aircraft
* An experienced instructor can make a difference compared to an inexp erienced one. Not to deride the enthusiasm or interest of a junior instructor - many are more enthusiastic than a long time bloke keen to leave - but experience develops instructor technique, skill & judgement. No different to any other field of skill.
Although my instructing was in Australia so whilst some fo the elements required to be learnt for any particular licence/certificate might be different, the fundamentals of learning are the same.
In Oz a PPL was able to be gained in a *minimum* of 40 hrs flight training i.a.w. the rules & regulations, however the typical mature age student required a median ofaround 60 or 70 hrs, allowing for the items I mentioned above.