The first time I took off my brush head was to change to a new one and I noticed that the actual gear part inside was skummy and moldy. Obviously I was not properly taking care of my Mia 2, and to think I was washing my face with this thing, disgusting! I looked over to my shower and relaized that my Clarisonic lived where it saw only moist conditions creating the perfect habitat for scum and mold. I definitely don't want to be washing my face with something that is unsanitary and gross. Because this is a little machine, you can't simply run it through the dishwasher for fear of over heating the motors or plastic. Here is a way I found that sanitizes as well as thoroughly cleans your machine head.
Take your brush head off of the machine, and lay this to the side. Taking hydrogyne peroxide (which is an antiseptic and kills bacteria), pour this into your Clarisonic head until it's full. Allow this to sit for a good five minutes. You will see some bubbling like you would if you put it on a cut, this tells you that it is cleaning any bacteria and left behind dead skin cells. After it's sat for five minutes, use a clean old tooth brush to lightly scrub away any scum. Then rinse this with hot water. Repeat this process until cleaned and let it dry out in the sun. You can let your brush head sit in the hydrogyne peroxide as well.