My third child was born unassisted, by choice. My first two births were attended by homebirth midwives; the first was badly over-managed, and the second was attended by a midwife who was very hands-off (I requested no timing of contractions, no dilation checks, and no coaching) and the experience was healing and empowering. It was then that I realized that I could, and (for many reasons) prefered to do it alone.
It was really pretty straight-forward. The pregnancy was mostly unassisted; I did my own health care and towards the end I had a midwife determine the position of the baby. I also acquired a fetascope and learned how to use it. The labor itself was fully unassisted.
I do know quite a bit about birth, as I have been doing self-study and research for the past four years or so, so I was confident that I would be able to recognize potential problems, but also confident that it was highly unlikely (given my specific situation) that any would occur, especially if my labor was allowed to progress completely unhindered. I did have two midwives who offered support on my terms, and it was definitely comforting to know that I had that option, but once the labor started I did not feel the need for it. Our back-up plan for emergency care was simply to go the hospital, just as we would in any other sort of emergency situation.
I did feel small twinges of fear at times, but each time I would examine myself and realize that there wasn't a valid source for it, and I would smile at myself and it would go away. Once the labor started I did not once feel afraid or doubtful of my decision -- my instincts took over and carried me.
I was a little afraid of well-meaning people interfering, and also, like chiromom, just didn't want to expend the energy on trying to explain myself. So before the birth, I only told people that I knew would understand or respect the choice. My mom, my brother, my mother-in-law, and my best friend. Everyone else just assumed that a midwife would be there, and if they happened to ask pointed questions I would just change the subject (I'm pretty good at that.
My husband was initially a little weirded out by the idea, but the more I shared with him my feelings and thoughts the more he felt comfortable with it. Reading to him from books and watching videos helped as well.