Russian Language.

[Разведка]

I agree - living there for one year taught me more than 15 years of study - and even then, I felt that there was so much I still didn't understand. Rosetta Stone has helped keep it from deteriorating too badly.

I found that reading Russian translations of my favorite authors or movies is also a fun way to practice - I've learned a lot of colloquialisms and slang that way. Vasha-Kniga.com - Russian Books in USA has some great stuff.

Best of luck, and have fun!