people who are bilingual are beter at multitasking, or more specifically; better at editing out irrelevant information and focusing on important information than monolinguals. Another study, by an independant research group showed that learning another language can help stave off dementia and alzheimers on an average of about four years.
I have always thought of the brain as a muscle of sorts. The more you use certain parts of it consistently, the stronger those parts of the brain get. The added benefit being added cognitive ability in whatever cortex you are working with (see this post).
If you don't practice a language now, why aren't you? There are plenty of people in the U.S. who speak a another language at home, 47 million to be exact. I know in previous posts I have suggested Livemocha and even Okcupid in order to find speaking partners. Your school or university is another place to look. I know the German department at my school offers kaffeepause every once in a while. Another site to look for conversational groups is meetup.com. By speaking with people in a casual environment, you will be without the stress of tests and quizzes that you probably remember from high school Spanish class, and you can work at a pace you're comfortable with!