Select increasingly difficult textbooks for your self-directed home study. "Kenntnisse: An Advanced German Course," by Claire S. Burke, offers students exposure to practical German for academic and career pursuits. The Foreign Service Institute's Mastering German program, meanwhile, offers students the chance to firm up their existing knowledge with a strong audio component.
Continue to study German in organized environments. As the material becomes increasingly sophisticated, it is helpful to have a more experienced speaker available to explain finer points as well as to make small refinements in your skills to help you transcend basic communication.
Interact with German outside of the classroom environment. Try getting your news from German sources (see Resources below). Turn off the subtitles when you watch German films to test your listening comprehension. Use bilingual stories to improve your reading skills and begin to read simple books all the way through in German.
Immerse yourself as often as possible in German-speaking situations. Speak German outside class with fellow students and visit German-speaking communities. To benefit fully from immersion, check into opportunities to spend time studying in Germany. When everyone around you is speaking your target language, your language skills will improve greatly and quickly out of necessity.