Here's the extended answer:)
It has been shown that being in an upright position helps to decrease the amount of pushing time, decrease the amount of tears, and decrease the amount of instrument deliveries. These positions include squatting, sitting, and on your hands and knees. There are good things and bad things about all of these. Obviously squatting would get tiresome, but if you have good help, they can help support you from behind. Hands and knees can also get tiresome, but it is one of the best positions to turning baby into the right position. You can also use a birthing stool to sit on, or if you are at the hospital, get the bed positioned to help support you sitting up. This is one of my favorite positions.
For those who have epidurals, or are just plain exhausted, the side-lying position has been shown to help. Essentially you have one leg raised while you are pushing on your side. With an epidural, you can also have the head of the bed raised to allow you to be more upright. Just make sure you are fully up and not just half way as this will decrease the space in your pelvis.
A good book to look at for positioning is Penny Simkins book, Labor Progress. She does a wonderful job of explaining different positions throughout the labor and why you would use them.
If you feel a need to be in any kind of position, I would stick with that. Many times, a woman's body can tell how they need to be to help move that baby down. Your biggest hindrance will be with medical staff that aren't used to these positions. I would make it clear from the get go that you want to push differently. Talk to your doctor about this also. Some are fine with doing whatever, and others are pretty stuck in the lying on the back mode.
I also had a question about doula's. First off, I'll say that I never had a doula, but probably could have used one. I have a wonderful supportive husband, but I feel like a doula who has been trained and been to many births, has experiences that neither I nor my husband had had at the time. Plus a doula allows the husband to take breaks when needed. A good doula will help the husband in his supportive role. I also think that there is something different about how men and women think, and having a women's support along with your husbands would make a difference.
Studies on doula's have shown that women who use doula's have shorter pushing times, less instrument deliveries, and breastfeed longer(Journal Of Obstetric, Gynecologic, And Neonatal Nursing: JOGNN / NAACOG [J Obstet Gynecol Neonatal Nurs] 2009 Mar-Apr; Vol. 38 (2), pp. 157-73.).
I would interview them first to see if they would fit well with what you want to do and how you feel about things. If you plan on having a hospital birth, I would also try and find a doula that has worked well with the medical staff in the past. If money is an issue, try and request a nurse that has had natural birth experience. Sometimes, you can get a nurse to help with the labor support if needed.
The Journal Of The American Osteopathic Association [J Am Osteopath Assoc] 2006 Apr; Vol. 106 (4), pp. 199-202.
The Journal Of Perinatal Education: An ASPO/Lamaze Publication [J Perinat Educ] 2006 Fall; Vol. 15 (4), pp. 6-9.
Journal Of Obstetric, Gynecologic, And Neonatal Nursing: JOGNN / NAACOG [J Obstet Gynecol Neonatal Nurs] 1997 Nov-Dec; Vol. 26 (6), pp. 727-34.