Read regularly. It is not particularly relevant what you read, but you should read books you find interesting and whose subject matter engages you. This will both expose you to new words and show you familiar words in another context, making their meaning easier to understand.
Write down words from your reading that you don't understand. This will give you a firm goal for words to add to your vocabulary and give you something to work on.
Repeat those unfamiliar words on your list to yourself a few times a day. This will make them more familiar to your brain and facilitate their use in conversation.
Apply your new words a few times a day by using them in conversation with friends, store clerks and anyone else. Use them in appropriate sentences so that they flow naturally rather than force them into a dialogue, which will make the vocabulary words seem unnatural and possibly incorrect.
Create mental pictures to go along with your new vocabulary words. Use your imagination, but you should be able to recall them quickly and always associate them with the vocabulary words you've assigned to them.
Play word games like Scrabble on a regular basis, and also do crossword puzzles as often as possible.