Reading is the key to improve your vocab. Read articles, blogs and various standard materials. When you read, you will come across various new words. Learn these words and look for their meanings. Reading is the tool to be proficient in vocabulary. Only mugging up the meanings won’t help. Try to learn atleast 5 or 10 words daily (according to ones suitability) and use them in your day to day life. Make sentences by using those words and use them daily. This can help you memorize those words more easily.
When you come across a new word, find atleast 3-4 synonyms and antonyms for those words and learn them. By this you will learn more words in a day. You can also try to manage a notebook and write all the new words and their synonyms and antonyms as this will help you in revision. It is very important to revise the words that you have learnt because its not easy to remember the words without revising them.