My wife and I used a doula for the birth of our son, and I would absolutely recomend it. We took three months of Bradley classes in preparation for a drug-free birth. I was mentally prepared for a 14-hour marathon, but our son was born in just under three hours, with only 20 minutes of pushing. Even being as well-prepared as we were, the presense of an experienced advocate was invaluable. We paid $300 for her services.
Anything that makes a pregnancy non-standard (twins, vaginal birth after cesarian, more than two weeks after due date) means that the hospital will be a bit more anxious to intervene to protect what they deem to be the best interests of the babies. You, however, may not agree. Some policies are negotiable, some aren’t. A good doula will know.
Take some time to educate yourself on what you’re in for. Learn the hopsitals policies on when they want to intervene, and when they’ll leave you alone. Most hospitals offer free classes that will tell you all about this. As a couple, have a game-plan for how you’re going to cope, and practice using those techniques beforehand. Then, share all of this with your doula. I can’t imagine our birth without our doula there.
Oh, and exercise exercise exercise. My wife was swimming everyday, right up to the day our son was born. And, it’s not part of your OP, but I’d recommend Bradley classes, if for no other reason than the emphasis on fitness and nutrition.