I've finally come around to a more holistic approach to chess study, openings included. This stems from developing a better understanding of the purpose I am looking to achieve with openings, namely to reach favorable (or at least equal) middlegame and endgame positions that I am comfortable playing against anyone, regardless of strength. (In contrast, some people like to try to win games in the openings, which is not my preferred style of play.) While my opening study therefore needs to incorporate the resulting middlegame characteristics and strategies, some openings can also lead straight to the endgame, so that should be an additional practical consideration as part of the opening selection and evaluation process.
Given this approach to the opening, the most natural and effective study method for me is to look at complete games with annotations that are heavy on conceptual explanations. The most useful openings book that I have ever read (and still refer to) is IM Nigel Povah's How to Play the English Opening - a 119-page book published in 1983. Somehow he succeeds in presenting the ideas of all of the major variations in the opening, provides lucid and valuable commentary on all representative games, and transmits a great deal of opening knowledge in one slim volume. It helps that the English is not full of forcing, tactical variations from early on - many of the games cited are still considered relevant to the practice of the opening today - but it is still an impressive achievement.
Close behind that book in my collection is Starting Out: The Caro-Kann by Joe Gallagher, which uses a similar method (as do others in the "Starting Out" series). The main difference for me is that I already had considerable knowledge of the Caro-Kann when I picked up the book, whereas Povah's was the first I had read on the English. The fact that I was still able to get a great deal out of Gallagher's book says a lot about the effectiveness of the complete game/conceptual explanation approach to opening study. Because I was already a practitioner of the opening, a large part of that involved validating my preferred lines and gaining better breadth and depth on my understanding of them.
Looking back on my history of openings study, it is notable how much I have retained and been able to use effectively the opening knowledge gained using the above method. In contrast, study which has involved sorting and memorizing variations without looking at many practical examples has been mostly useless in the long term. On an intuitive level, if I don't understand the moves and their consequences, why should I be playing them? This realization has been something of a breakthrough for me, as for some reason it often seems easier to closely imitate others than to rely on your own openings work.
The silliness of completely relying on others for your own opening evaluations is regularly demonstrated as everything from sub-variations to entire openings can come in and out of fashion relatively quickly, while true masters use openings as weapons regardless of the opinions of others. Magnus Carlsen pulled out the Philidor Defense in order to guarantee his recent victory in the super-strong Biel tournament, while Garry Kasparov wasn't afraid to resurrect the Evans Gambit and Scotch Game, both of which had been written off at the Grandmaster level long before.
I have to say that I enjoy what goes into becoming my own openings expert, in part because it is something of a creative process. Although I use opening books and occasionally DVDs as foundations, that is really just the start, as other possibilities are always revealed using games database and chess engine analysis, combined with one's own critical thinking. Every time I step through a portion of my own openings book in this manner, I always end up refining things, typically choosing new sub-variations after examining the computer evaluation and running through various high-level games in the database. Similarly, analyzing my own games tends to reveal new opening possibilities and improvements, for example in Annotated Game #3. I believe that further deepening this opening study process will lead to strengthening my entire game, as the middlegame and endgame possibilities unfold and are better understood.