There are lots of tips out there for ensuring an adequate supply of breastmilk: drinking dark beer, eating oatmeal, taking herbal concoctions. Sure, these may help, but there is one thing that is the most important to ensure an adequate supply of breastmilk:
Allow the baby unrestricted access to the breast. The more the baby nurses, the more milk the mother will produce.
Supplemental feeding with formula, especially during the first 3 months of breastfeeding, can inadvertently sabotage a mother's milk supply. Additionally, bottle-feeding with pumped breastmilk can reduce the mother's milk supply. Why?
Mother and baby have been perfectly designed to make breastfeeding work. The more the baby stimulates the breasts, the more milk the mother makes. The more the baby is given something other than breastmilk, the less the baby is stimulating the mother's breasts, so the less milk that is produced.
Use of pacifiers can also result in a reduced supply of breastmilk. Pumping breastmilk does NOT stimulate the breasts the same way that a nursing baby does, so pumping rather than nursing can also reduce milk supply.
I experienced this myself with my firstborn since I was still working outside the home. I pumped breastmilk for about 15 months so that my daughter would be able to drink breastmilk while I was at work, but it was very obvious to me early-on that I was not able to pump very much. If my daughter was nursing, the milk would just keep coming as long as she nursed (as evidenced by repeated let-down sensations and her vigorous swallowing). But with pumping, I was only ever able to get a small amount of milk at a time.
What does unrestricted access to the breast look like?
Unrestricted access to the breast is simply allowing the baby to nurse whenever it wants to. No feeding schedules. No limitations. No timed feedings. Like many, I wanted to micromanage feedings with my firstborn by counting the hours between feedings and making sure the feedings lasted a certain amount of time. How else would I know she was getting enough?
Micromanaging the baby's nursing is actually unnecessary. The baby will naturally nurse whenever it needs to, and can actually increase the mother's milk supply whenever more milk is needed. Trying to set an artificial feeding schedule for a baby can result in less milk production, and it is not worth the time and effort. I had learned this lesson by the time I had my second child, and it was so much more calm and relaxing to just let my son nurse whenever he wanted to.
What if the baby seems hungry and wants to nurse constantly?
There will be days when the baby wants to nurse all day and seems very hungry. This does NOT mean that the mother's milk supply is inadequate! This is nature's way of increasing the mother's milk supply to match the baby's needs. Breastfeeding mothers need only take the time to relax on those days, allowing the baby to nurse as much as it wants. In this way, the mother's milk supply will increase and the baby's need to nurse so frequently will diminish (until the next growth spurt, anyway).
Other ways to ensure an adequate milk supply
Co-sleeping and babywearing are two more ways to establish a good milk supply. Both of these practices encourage unrestricted nursing due to the proximity of the baby to the mother. With both of my kids, these two practices were also the best for keeping my babies content. They loved being held much of the time, and both loved to snuggle with their parents while sleeping. Co-sleeping also makes middle of the night feedings much less disruptive to sleep, since all that is required is to roll over (rather than getting out of bed, warming bottles, etc).
Want to learn more? The following resources are great for nursing mothers:
So That's What They're For!: The Definitive Breastfeeding Guide is my favorite book about breastfeeding. It clearly explains the many benefits of breastfeeding and the mechanics of breastfeeding. It is also great for trouble-shooting any breastfeeding problems, and includes sections on going back to work, weaning, and toddler nursing.
The Deep Latch Technique is great for mothers who are experiencing any discomfort beyond the initial days of nursing. I used this technique with my second-born when I found that I was developing scabs due to his improper latch position. I never realized I'd have to re-learn to nurse with my son, since he nursed so differently than my daughter.
Kellymom.com is an amazing source of information for breastfeeding mothers. It contains a wealth of information on every breastfeeding topic, from proper weight gain, to low milk supply, to overabundant milk supply, to pumping.
Formula Harms Mothers is a great article that details many of the breastfeeding benefits for mothers. This is a great look into how breastfeeding is good for mothers in particular.
La Leche League is a wonderful resource for women who have breastfeeding problems. In most locations, they have local chapters with people available who can help troubleshoot breastfeeding issues free of charge.