this is actually a good question - i use just the camera on my phone and it is a freaking pain in the neck to take a picture of the top of my head. Cameras/lenses have minimum focusing distances; if you are using a regular DSLR:
1 - set it on a tripod (and leave it in one place so lighting is consistent
2 - focus it on a chair x feet away from the camera (# of feet will vary depending on lens)
3 - after focus is achieved, set the camera to manual focus (so the camera doesn't change focus point when you actually want to take a photo)
4 - set the camera to self-timer and press the button
It's going to be a pain every time - unless your DSLR has a swiveling/articulating LCD screen that turns all the way around then you'll have trouble taking a picture of yourself without setting the camera on a tripod and roughly following the steps i mentioned
I was actually thinking the other day that it would be a nice idea to make an app that is basically a "hair tracker" app. You would open it on your phone, it would recognize your face and take photos plus stamp the date and prompt the user to capture the different angles - then you could export a "timelapse" video of the progression of the hair, etc. I think that would be pretty cool...