Synonyms - are words that mean the same thing. When you have learnt a new word see if there are any synonyms for it. It expands your vocabulary and it will make your English more interesting and less repetitive.
There are several online dictionaries that can help: Thesaurus.com | Synonym.com| Rhymezone.com
Yet again using flash cards as a memory aid can help. Write the word you know on one side of the card and the most common synonyms for that word on the other side. See how many you can remember.