I wrote quite a detailed birth plan because I had some set ideas on what I wanted and didn;t want and it was all followed to the letter, although I don't know how much of this was because of the plan and how much was just that it was all common sense.
I said I didn't want pethidine or an epidural unless absolutely necessary and near the end when I was asking for an epidural, the midwife got me breathing the gas and air more regularly and that took the edge off enough.
I also said I didn't want to be lying down and wanted to move around to find the easiest position. In the end I didn't feel like moving round at all and the only position I felt comfortable in was sitting up straight on the edge of the bed. I don't know what I was thinking of because there was no way I could have given birth like that but the mw encouraged me to kneel on the bed leaning on the head of the bed, which was raised. Since that, I haven't been able to think of another position that would have been comfortable for me so that was well done.
I said I wanted syntocin given straight after the birth, so they did that while I was still getting over the shock and didn't notice at all. And so the placenta came out really quick and while I still had a tiny bit of energy left!
I'd also said that I wanted the vitamin K given as an injection to the baby but they asked me again what I wanted at the time, which I thought was good, in case I'd changed my mind after seeing dd or something.
I think generally midwifes are very careful to try and follow a birth plan but there's no harm in writing one, even if you think it might not do a lot of good. And if your DH has a copy on him through the birth and is very familiar with what you want, then he can prompt the medical staff if they're not quite doing what you want.