I shave my head 1-2 times weekly with a straight. I use a hand held mirror + bathroom mirror to do the back. I too have found that it does dull the razor quite a bit more quickly than face shaving. On the plus side, I have found that pre-shave is not really important in noggin' shavin'.

I like 6/8' blade for the head, as there is more surface area to cover and you wont' have to wipe the blade as often as you would with a narrower blade. I usually do one WTG and one ATG pass. Consider a stroke or two on a barber hone after each shave to keep your blade extra sharp.

I like to use some witch hazel, then glycerin + water, or moisturizing aftershave after. Like mentioned above, head cuts tend to bleed pretty good, so be prepared for that.