"efficacious"? I had to look up that one. Well, usually for common English vocabulary improvement, I would suggest watching BBC and reading, reading novels and the top selling English newspaper if you are in a non - English speaking country or just famous journals (national and international).
So there is only so much you can improve your vocabulary from sources that I mentioned above, and they are the most common sources most people turn to, when it comes to improving their English vocabulary. Just like you used "efficacious" in your question which is not very commonly used (In my opinion, I might be wrong or not well informed), I would have just used effective instead of "efficacious", there are so many words which I consider to be tedious or overkill to represent/communicate simple ideas and efficacious seems to fit the bill.
But in your case, you are preparing for a standardized test and these tests always require you to memorize and understand so many words like these(Totally unnecessary, you tend to never use half of the words , ever in your life , or end up forgetting most of 'em after the test is over, IMHO of course). So there are always good books, just for the purpose of teaching you words related to that exam/test. The books written just to prepare you for the test is your best bet, because improving your vocabulary of the sake of acing the test is a specialized circumstance, because the common sources like TV channels, newspapers and journals are aimed at the general population, so it has to be simple and concise so that everyone can understand it and not loose interest as well, so they wont work for specialized circumstances such as a standardized test. So in conclusion I would say, get a good book that helps you prepare for the exam.