My first thought is that every labor is so different, every woman's body and every baby are so different -- and the factors that will affect ever labor vary so much...
BUT given all that, lol
I would echo what Pam said (esp about active/upright!) and add that I believe fear will almost always slow or stall labor. A situation in which a woman feels both physically and emotionally safe and supported will create the optimal environment for labor to progress most smoothly.
Oh, and one other thing -- I've heard many a second-time mom whose labor went "much faster" who didn't necessarily prefer the speedy & intense experience to the gradual building of the first time around!
One great book on positions, etc is Janet Balaskas' "Active Birth."