My first real attempt to learn Mandarin was the summer of '98 after my freshmen year when I took a ten-week intensive course in UC Berkeley Summer School. That was a miserable attempt as the pace was ridiculously fast. I didn't retain much from that summer.
My next attempt came after graduation when I went to National Taiwan University in Taipei, Taiwan for the summer and fall of 2001. I stayed for almost 6 months and learned a tremendous amount. My learning was not limited to Mandarin, as I learned a lot about life outside the United States as well as life in general.
Since returning from Taiwan, I have kept up w/ my Mandarin through Berkeley Extension, tutoring, and self-study.