Read articles and other media in German. Even if you don't understand everything that you read, you will learn new words by reading them in the context of words you do know. Notice words that often are used together or in certain genres of writing. Look up the words in a German-to-English dictionary.
Listen to German as much as possible. News broadcasts and other radio programs are good places to start. Also watch German videos on http://www.YouTube.com. There also are plenty of videos on YouTube to help you learn German. Complete a search for "learn German."
Join an online language exchange such as My Language Exchange (http://www.mylanguageexchange.com). You will meet with a German speaker who wants to learn English. You can pick up new vocabulary as you converse with your language partner, and you can ask questions about new words that you come across in your reading and listening practice. By speaking in German with your language partner, you will reinforce the new words you learn.
Write short documents in German once or twice a week. Compose and send an e-mail in German to your language partner or write a short story about your life and ask your language partner to help you correct it. Use the words you have recently learned. The physical act of writing the words down will help you retain your new vocabulary.