Follow the news. Information about what is happening in the world, why it is happening and the background behind current events is excellent knowledge to have in your repertoire. Read a daily newspaper either online or in print in order to keep up to date. Watching TV news can help too, but TV tends to give less background about events than a written report would.
Read classical literature. You will better understand various cultural references if you have read commonly cited works such as Shakespeare and Edgar Allan Poe, and having these books in your knowledge library will broaden your perspectives.
Read philosophy and science, or anything that makes you think and want to know more. The process of building new mental connections and expanding your horizons will directly affect how much you feel you know.
Develop and answer questions. If you have ever wondered about anything such as why food allergies develop or why the stock market crashed in 1929, search the Internet to find articles that answer those questions. Get into the habit of finding these answers rather than simply wondering why and then moving on to something else.
Take courses. Even if you are not studying for a degree, look into evening or online classes at your local community college. If there is a class you think sounds interesting, take it just because you want to know more.
Get into discussions with others. Hearing other people's ideas and responding to their questions about your own will give you a better understanding of other people's perspectives and can broaden your own in the process.