The easiest way to improve vocabulary are the following that could be effective for a person who wants mastery over English language in day to day life. These methods can be practiced by anyone without any age limit.
1. While you travel read the banners and signboards and whatever you come across.
2. Try to find out words for things you find in kitchen.
3. Use the new word in your communication may it be spoken or written.
4. Read the names of various dishes in the menu you find in a restaurant.
5. Listen to talks that you find interesting. Note down two words that you could use in your daily life situations.
6. Buy books of the authors you like to read and start reading. Note words that are new to you and use them.
7. Find synonyms and antonyms for a particular word.
8. Watch travelogues as you may get many new words.
9. Talk to native speakers when you get an opportunity.
10. The key to building vocabulary is just exposure.