I took German for two years, and am now spending a year abroad in Germany. Been here for five weeks, and will start school tomorrow. After only a month, my German is much better—I understand much better, speak faster, and know more. There are words and phrases I use everyday that I never learned in school, and things my teacher stressed that I never need. My progress has proven to me that the best way to learn is to immerse yourself in the language. I am still far from fluent, but I get better everyday.
Some helpful links:
How To Learn A Language,
German Wikibook
I also reccomend getting a grammar book once you get to the intermediate level, and learning something new every couple of days. I have Langenscheidt’s “German Grammar in a Nutshell” and love it.