New stuff added to reply:
In case I hadn't put everyone to sleep with my original post (yes, I know I can go on a bit!) I thought of something else I wanted to say.
When I began exclusively pumping for my son, I knew next to nothing about lactation, despite having breastfed my older son. Maybe I was just asleep on the day everyone else learned this stuff, but since I found it useful I wanted to pass it on. Also, I enjoy making incredibly long posts :-)
Before I started reading everything I could to help me EP for Nick, I had a vague idea that milk production was a constant thing -- a mother's body constantly produced milk, which would be stored in her breasts until her baby nursed and emptied said breasts, and then the process would start over. Turns out that isn't the whole story. Your body does constantly produce some milk, which is stored in the milk sinuses in your breasts, but the average breast can only hold an ounce or two of milk. When the baby starts nursing and the sinuses begin to empty, the body is triggered to make milk on the fly. Most of the baby's meal comes from the made-on-demand milk, not the relatively small amount that had accumulated in the mother's milk sinuses. Some women experience this instant production of milk as a noticeable lowering of hydration and blood sugar levels -- it can be a good idea to have a drink and a snack nearby. The point is that you don't need to wait until your breasts 'fill up' before you can nurse or pump again. In fact, waiting until you feel engorged will send a message to your body that it is producing too much milk, as will nursing or pumping only a little bit of milk at a time. If you want to stop lactating, wait until you are engorged before expressing just enough milk to feel comfortable, avoiding going into instant-milk-production mode. If you want to increase suppply, pump before your breasts feel full, and pump for as long as your body can keep up the instant production. This will tell your body that more milk is needed, and the amount of milk your body can produce on demand will increase.
Okay, I think I'm done now :-)
Original post:
I pumped and bottle-fed my younger son. Due to a cleft palate he couldn't nurse but I was determined to give him breastmilk. The key things are using a good pump and pumping often. No pump is going to provide the physical and emotional stimulation of a suckling baby so you have to compensate by pumping a lot!
If you're using a consumer-grade pump I strongly suggest renting a hospital-grade pump. Even the best consumer pumps such as the Medela Pump In Style are not as good as hospital grade pumps. After you have established your supply you can use a consumer pump, but at least for the first couple of months you should use a hospital pump. I rented a Medela Lactina. Medela have a newer hospital pump out called the Symphony which I have heard good things about. Make sure that the attachments are new and replace them if they start to look worn. Even if it looks okay, replace the little yellow valve and white membrane thingies every month -- the membranes lose their flexibility with use and this has a huge impact on suction.
You need to pump often. At first I pumped every two hours for 20 minutes. Yes, even overnight for the first week. Yes, it was a total PITA. You want to pump more milk than your baby needs to be sure of getting enough stimulation. Buy milk freezer bags. I found the Gerber bags better than the Medela ones, because the Gerber ones have a ziplock closure and they can be frozen into a flat stackable rectangle, which saves space in the freezer. After the first couple of weeks you can reduce the frequency of pumping. The optimal frequency is going to vary from mom to mom, and may take some experimentation. You need to pump often enough to produce more than your baby needs, enough to freeze a couple of freezer bags every day.
Pumping less than your daughter needs, and making up the gap with formula, will result in your supply diminishing. Initially, as your milk comes in and your build up your supply, you may need to supplement with formula, but if you want pumping to work long-term you have to work toward an oversupply. The formula itself isn't the problem (of course, breastmilk is much better, but formula provides perfect adequate nutrition), the problem is that the less you pump, the less milk your body will produce.
I was lucky enough to be able to be a stay-at-home mom so I could plan my time around pumping, and I was blessed with productive breasts :-) so I did well, but I won't pretend that exclusive pumping is easy. Having done both, I can assure you that nursing is MUCH easier! I was on an EPing support group, and plenty of moms had work commitments that prevented them from pumping, or bodies that just couldn't get all relaxed and maternal and milky about a plastic-and-metal machine. You didn't say why you had to pump instead of nurse, but whatever the reason I applaud you for trying. The first month of breastfeeding is the most important as far as the health benefits go -- yes, longer is better, but the rate of return diminishes -- so if you find that you can't keep up the pumping, please don't feel bad about it. You're doing a wonderful thing and you should be very proud of yourself.
If I can be of any help, please let me know.