I was quite surprised to find out that even if you are an organ donor, they won't take your brain. Even if you've checked the box off on your driver's license. In fact, there isn't currently even a central repository to make your wishes known. However, you can at least let Harvard Medical know. Fill out form here: http://www.brainbank.mclean.org/register.htm. FORM
I recommend this article. Brains. Brains!