Learn from my mistakes and avoid these habits if you are attempting to learn German. Your life will be that much better for it, and so will be your mastery of the language.
1. Compartmentalizing German. If you delineate it as "this is German learning time for X minutes and then I'm done" instead of trying to incorporate it into your daily life, you won't make a lot of progress.
2. Trying to be perfect. It's never a bad idea to think about grammar and proper usage, but your conversation mates might grow old if not bored while you try to perfect all of the articles in your head before you speak.
3. Going back and correcting a sentence after saying it (unless it was understandable or it didn't have the meaning that you intended). This totally breaks up the flow and acts like a verbal red pen. Just keep going!
4. Constantly trying to directly translate idioms. That will only make your head hurt.
5. Always hanging out with people who are the same nationality as you are or who speak the same major language(s) that you do. Even with the best of intentions, you'll most likely end up speaking the majority language that is easier, all the while not practicing German. Of course, it can be difficult to find people who speak no English, for example, so try making a pact with friends to practice a lot of German.