I was not in as good of shape as I would have liked while preg, but I still think I did ok. PP suggested DVD for yoga at home, I'll second that, I did some, wished I would have done more. And kegals! Do them everyday.. seriously. I was lazy about that until the first time I sneezed and lost bladder control... I did them everyday after that so it wouldnt happen again.
I think the most important thing though, is a positive attitude about it, and MAJOR emotional support. DS was posterior, and my labor was much more painfull than I had anticipated, and I was so tired near the end when it was the hardest, I had a 34 hr labor. I almost broke down near the end and asked for an epi, but DH got me through it by reminding me of all we had gone through so far to acheive a natural birth for DS... he had been breech, we had done weeks of trying re-position him, and spent a few hundred dollars for a chiro.. all the weeks we spent interviewing MW's, all the practicing we had done, etc. He reminded me of why it was so important for DS to be born w/o drugs. Luckily he did this in a very calm, loving way... if he had at any point been non-supportive or dissmissive of the pain I was in while he was talking to me, I would have lost it... I was in such a raw and emotional place at the time.
It is just SO VERY important to have the right kind of emotional support, I really cant stress that enough. Compared to that, the kind of physical shape you are in, it just doesnt compare. And you get such a rush of energy when you start pushing anyways. (Pushing is SO much better than labor, ITA with PP!! ) Even after that long labor, I remember being in our room a few hours after the birth... DH was exhausted from doing counterpressure for me 11 hours straight and passed out cold, but I was so awake and energized and estatic! One of the nurses said I couldnt possibly have just given birth while she was examining DS, because I was smiling and laughing and up walking around the room with him.
Good luck Mama!