PPs have really covered the big stuff (what if I transfer or need a c-section??) but I think this question come in to play about the small stuff too (will it be painful or not? will I be in water or not?).
I had an amazing, awesome, powerful, transforming homebirth in October. I prepared for it the best I could. I researched birth, midwives, the intervention rates in hospitals. I looked at common birth problems and what is usually done about them. I read birthing books, took classes, watched BBOB and as many youtube homebirth videos as I could find. I talked with every woman I could about her experience. I stalked the MDC boards like a fiend. I gathered the supplies and envisioned my birth. I planned pain management techniques, cooked food I wanted to eat. My husband was an amazing support person. I had a confident, strong, unafraid mindset.
I felt that I was prepared, informed, and had chosen the place of birth and the care providers that were absolutely best for me and my baby. I picked the safest place (for me) and did my best to ensure I was not in the position to get *unnecessary* interventions.
Beyond that, I realized that I could *prepare* for birth, but I couldn't really *control* it. That is, I could set myself up to have the best possible circumstances and factors for a "good" birth (i.e. the birth I was planning in my head), but I could not actually ensure that a "good" birth would happen (turns out I did have a great birth). I really think that's the key. I think you (general you) do your best to make sure you put as many positive factors in place as you can and weed out as many risk/negative factors as you can. You give yourself the best possible chance of having the birth you want - but it doesn't mean you can guarantee it.
As for the "small" stuff: I was very careful to keep an open mind about how things would go. While pregnant, I mentally prepared myself for a painful labor or a pain-free one. I thought about a long labor or a very precipitous one. I had a labor tub but kept open the possibility that I would not want to use it (as it turned out, baby come to quickly to use it). I told my husband I could envision massage being helpful, but could also see not wanting to be touched at all during labor. Basically, in all aspects, I tried not to get too wedded to "how things would be."
As an example, I did keep thinking I would deliver my baby in the hands-and-knees position. For some reason, I had it in my head that that would be the "best" position. Turns out, hands-and-knees made contractions *unbearable* and I ended up laboring and delivering standing up. For awhile though, I kept trying to force the H&K position, until I consciously told myself to let it go. Then I was able to find the position I really needed to be in.
This post is starting to ramble, so I'll just sum up by saying, prepare yourself the best you can, set yourself up so you have the greatest chance of having the birth you want, try to stay flexible about the details, and realize that things may not happen exactly how you plan.
