There are many ways. But some of the others suggested are good. I think just do anything in English that you enjoy- movies, books, songs, try language partners. The main thing is not to look at it like a "job" or a "task" but as something like a hobby, something for enjoyment. THis way you will quickly learn more vocabulary items and also have fun.

In my opinion you should focus on writing idiomatic English. That means English the way native speakers use English. You don’t need a very big vocabulary in order to do that. What you need to do is to use the words you already know in a way that sounds natural in English. For instance you write “Although my English is gradually improved”. The correct sentence is “Although my English HAS gradually improved”. You have to learn to crawl before you can walk. Learn to write clear, grammatically correct sentences that sound good. In my opinion, a good essay is first of all an essay that shows you can think analytically. That should be your main focus. You can polish a turd, but it doesn’t change the fact that it’s a turd. If your teachers or potential readers think otherwise, they are in my humble opinion fools. “Are there any ways to remember new words fast” In my opinion, as an advanced learner of English, the answer is no. You will remember words that you use regularly and that you are exposed to regularly. There is no trick.

Man,just watch movies,read books and write out unknown words and phrases to special log and then work on them.just con them.The i read the books,sometimes i am capabele to catch some reapeating words fom time to time,but most of unknown words usually are left forgotten.I keep my log and put every unknown word into it and after reading i con then.I think there is no easy way to expand your volcabulary,there is a thousands of words with you dont know and all that you need is time and patience.I have the same problem with my volcabulary and every day i start to wonder again and again how rich is English language is.