I've been shaving my head for about 10 years now and have used everything from the Mach III to a ladies' bic to a really nice Norelco electric my wife got me for Christmas last year.
The best way to go, imho, is the Mach 3 in the shower with some good quality shaving gel. I keep my head under the hot water for a few minutes, apply gel, and get to it.
As far as dry head, I use a scentless lotion and I'm good to go.