Memorize the dictionary, you say? :(
I don't think so! Some time here and there, maybe, nosing around in the dictionary - but don't overdo it! Sometimes dictionary work is good for finding interesting new synonyms or antonyms, but it doesn't often help you much as far as appropriate idiomatic usage is concerned.
Read, read, read. And then read some more! You don't have to stop at every unfamiliar word. Most of the time you will learn words by context. You may want to write down a few interesting words to look up later.
Choose the widest variety of reading materials possible -- everything from classic novels to daily newspapers. Every subject matter area has its own vocabulary.
Write unfamiliar words (or better yet, phrases) on vocabulary cards - English on one side - your own language on the other. Drill them every day.
I'm sure others will also offer their suggestions.
![Smile [:)]](/emoticons/emotion-1.gif)