Serack, i had an idea about this too.  I was thinking of doing a re-read and picking up how Harry's descriptions of using his power change.

For example ( i am at work and don't have the books, so this is just my trying to expalin what i mean)
He may describe in an early book how doing a particular spell takes all his energy and concentration.
In a later book he may say the same or a differnt spell is easy or second nature.

This to me is also part of his power ups.  How things are hard in the begginning and become easier or take less concentration or the spell itself has so much more power in it.

Plus i tend to read fast and not soak in details and it would force me to pay attention.