I guess I'm in the minority, but I actually did better on the MCAT the second time around.
I originally took it in 2001 and was fairly satisfied with a score of 32M (10P, 11V, 11B). However, due to circumstances beyond my control, I haven't been able to apply until now, and those scores are already too old for some of the schools on my list. So I was forced to prepare for and take the test again this past April, when I managed to bump my score up to a 35P (11P, 12V, 12B).
Obviously the statistics were not in my favor, but I can honestly say it's possible to improve, because it happened to me. I must caution you that it required a HUGE commitment of time and effort on my part. But if you really want it, then there is hope.