Hi, Leo.I also agree with you that if we lack of vocabulary, it is difficult to express all the ideas that we want to say. Now, I am also try to improve my english vocabularies day by day. In my opinion, you can find the english vocabulary book which is divided to many topics. Then you plan the learning schedule such as one topic for one day. To me, i use the "English Vocabulary in use" book of Cambridge. I find it is very useful for me. Besides, you can also use the good English dictionary to enrich your vocabulary such as Longman, Oxford, etc. All these dictionary will help you to distinguish the usage of each word. You try to read the meaning carefully and some example sentences to find out how differences they are. If you have already understand the usage of the word, you will seldom be confused. Above is the materials for studying vocabulary. About the method to remember new words in heart. You can also use the notebook to write down the new words includes the meaning in English, the family words (Adjective, Verb, Noun, Adverb), synonym and antonym of the words and collocations (words that commonly go together). You can also use the tree-diagram, word-map,pictures etc. to organise or link the word in groups. By the way, if you want to learn 20,000 words, you can make a plan to learn 5-10 new words per day or more than if you can remember :). Then after one week, one month or several months i belive that you will reach your target or at least own a rich vocabulary bank in your mind. These are some advices for you. Hope that it will be useful. ^^
Meisha.