I Learned to Speak Four Languages in a Few Years: Here's How
Once you master one foreign language (in my case my native language is English, but I'm fluent in German as well), try to learn another foreign language through that first one. For example, I'm currently studying at a German university, and I'm taking a beginner Russian course. Because I'm at a German university, the course is obviously taught in German, and it's a really interesting and helpful experience. You've already gotten pretty good at the foreign language in itself by this point, but learning another language from that first foreign language really helps to polish off your language skills. I'm sure it's also a good workout for your brain trying to deal with 2 (or more) foreign languages simultaneously. Also I can't help but reiterate some of the other advice given here in the comments: nothing can replace extended immersion in a country in which the target language is spoken.