I think the most effective way to do this in a short period of time is to get a book on word prefixes, suffixes, language origins, etc. This is more efficient than just memorizing huge word lists.
I can't remember the title, but when I studied for the SAT I had a great book that discussed all the language origins of many english words. Once you get these down, guessing is quite easy. (Most "learned" words are not really English at all.)
Also try something like Verbal Advantage. It's much easier to read than the dry lists printed in Kaplans, Princeton, etc.