I took an ice bath once in high school the night before a cross country race. Supposedly, it helps flush lactic acid out of your system. First, you take the ice bath for about 10-15 mins. Then you immediately get into a warm/hot shower and your blood really starts flowing again, flushing it out.
Not sure how true that really is, but it seemed to help me - I set a big PR the next day. Of course, I never did it again. That fucking ice bath was tough.