I have been at births where mom has pushed for hours upon hours. Never forced, never flat on her back unless by choice (and yes, some moms prefer that position sometimes), never in one position for very long.
If a mom has a long labor, sometimes the uterus is just plain tired. Resting in the tub, keeping well hydrated, eating some sugar (a spoonful of honey will do wonders, if mom can handle the sweetness), and patience usually work.
But also remember, if mom has contractions 3 minutes apart, lasting for 1 minute, and she pushes for one hour, well, she is spending the same amount of time pushing as someone whose contractions are six minutes apart, with contractions lasting about 30 seconds, who pushes for three or four hours. The spacing between the contractions and the length of the contraction also has alot to do with how long mom pushes.
I think that many times, long labors happen for a reason. I think that sometimes a mom or baby can't tolerate a short, hard labor; due to positioning, cord involvment, maternal blood pressure, whatever. The body is pretty wise, and usually gives you what you need.
Likewise, I have seen labors suddenly kick into high gear, and later the reason becomes apparent. Most vivid in my mind is a labor, second time mom. She was progressing nicely, feeling good. She asked to be checked, she felt like something was happening. She was at six, and the baby was at +1 station (still had a ways to go, but was pretty low in her pelvis). Literally the next contraction, she felt like pushing. A huge, enormous, overwhelming urge. We couldn't have stopped her if we tried. She resisted for a contraction, then the mw did another exam (we were all wondering exactly what was going on, including mama), and the cervix was almost gone and the baby had moved down significantly. And baby's heart tones start to go down. And we start to see some blood flow, a little more than one would expect; but we thought, well, she is dilating quickly, there are probably some vessels being broken; also we feared a cervical tear. We discussed transport, but things were moving so fast, there was no way we could have gotten her to the hospital. Three contractions later, with slowing descending heart tones and slightly increasing amounts of blood, the babe is born, with the placenta practically between his legs. The mom was abrupting, meaning her placenta was coming off the uterine wall too soon. Her body suddenly realized that there was going to be some very serious conseqences if that baby wasn't born right then, and it did the job. It was incredible. It totally sealed my faith in the power of the human body and in the amazing strength and knowledge our bodies hold. I mean, that mom could have bled to death, the baby could have died; but something very primal and amazing took over and protected her; she listened and went with it.
So, I hate to see us mess with labors that are slow, in the absence of any reason that would cause a slowing of labor (fear, dehydration, malpositioning of baby, etc). I feel like maybe something in the baby is sending signals to the mama's body, saying "Hey, keep it easy on me, I'm not up to hard and fast." And when we mess with that, we are really disrespecting the body's wisdom and instinct.
I also dislike our culture's feeling that labor is something to get over with, a horrible thing you have to go through to get your baby. The same way we disrespect our uteruses when we talk about how awful it is to have a period, "the curse." Urgh. Labor is certainly no walk in the park, but it is an initiation. It should be empowering, a woman revelling in her strength. Husbands/partners rarely fail to be utterly in awe of the entire process, and I want women to feel that way about it, too. To see how incredible it all is, how well her body and mind can work together to bring this new life into the world. Birth should be a celebration of new life and of the woman's emotional and physical power.
Okay, I'm off my soapbox now. I get carried away with it all sometimes....