You have to use different sources to enrich your vocabulary...
Prepare separate notes for the same- one in alphabetical order and other as general vocabulary (i e taken from newspapers, articles, blogs, books, etc).
Do not only note down words on paper but also use them in the sentences of your own, so the words get embedded in your mind once and for all.
Oxford Advanced Learner's Dictionary is the best of all as its explanation is very easy to understand with clear usage of words in every possible context. It provides the idioms and phrasal verbs ( if there are any ) of a particular word you are looking up. Here is the link to refer this dictionary online on internet:
Oxford Advanced Learner's Dictionary at OxfordLearnersDictionaries.com | Find the meanings and definitions of words at OxfordLearnersDictionaries.com
GRE Worldlist, SAT and Barron's Vocabulary Builder work and prove a great help too.
Some seniors above have greatly communicated their help by recommending books, online internet sites, etc... Do try them all as well...
Hope it all helps