Dear Mama-to-be, YOU know best (really)
The best advice I wish I had taken fully to heart during my prenatal class was to be an unabashed advocate for myself. I intellectually understood the importance of this. I wanted to take ownership of my pregnancy and labour. I certainly knew that no one could read my mind so it would be up to me to share my needs and desires.
I didn't understand that there would be strong forces to contend with (especially during labour) and a very short amount of time to respond. The best thing you can do is think through scenerios in advance and plan out your ideal response. That way, when you are challenged with the reality of how things go, you hopefully have some idea which way to respond.
Quick example: I was in labour and things were not progressing quickly. Baby was fine. I was fine. But things weren't moving along. The nurses suggested I could have my water broken to speed things up and I liked this idea. Then the doctor came in and spoke persuasively that an IV drip of oxytocin was the superior first-step (and we could break the water later.) He seemed intelligent and confident, so I agreed.
It took FIVE attempts and multiple hours to get an IV into my veins. And then after it was finally in... things still progressed slowly!
Finally, a new doctor came on shift and offered to break my water. This time I said yes. She did so, then she walked out of the room to check on other patients. Less than ten minutes later my baby was born with only a nurse in attendance (the labour progressed faster than anyone would have guessed! The doctor didn't even make it back to my room in time.)
My original instinct was to break the water. But in the face of confidence (the first doctor), I surrendered my idea to the "expert" opinion. In retrospect, I believe that my baby would have been born much sooner if I had originally stood up for what I wanted (also, I wouldn't have been unnecessarily poked with IV needles.) I will also point out that the only ones who had a problem with the "slow" progress of this labour were the health care professionals. I was content with how things were going until it was suggested we could move things along with an intervention (ie. breaking my water and the IV drip of oxytocin.)
In summary, my advice is to gather lots of stories so that you can imagine yourself in a variety of scenerios and think through what you want and identify what is important for you. Be sure to communicate these insights to the people who are likely to be beside you when the moment of truth comes along (for labour, that would be your birth partner.)
Resource: the book "Ina May's Guide to Childbirth" has a number of labour stories that are very easy to read and give a good idea how diverse labour can be from birth to birth!