daisywheel
Lots of practical usage of language is essential to grow the vocabulary. This is long term strategy.
My question is focused on short term strategy and I look for your hints.
During the preparation to GMAT you might meet unknown words which lead to misunderstanding of passage and wrong answer as result.
Do you collect such words to review them letter on? Do you use cards or two-column dictionary or something else?
If you're discovering a LOT of new words each time you do a verbal set, you might want to get an SAT prep book from a library or bookstore. They should have words that commonly appear on the SAT, and in many cases, on the GMAT/GRE. You can then review those via flashcards.
If you're discovering new words in the RC sections, and they're highly technical or context-specific, just make a list of those words. Review them using flashcards shortly after your prep session. There shouldn't be too many of these though...