Just like the other posts, hydration is essential! I once calculated it out, and while I was pumping I drank a minimum of 96oz from the time I left the house in the morning to the time I came home at night! 32oz of that would be either Mother's Milk Tea by Traditional Medicinals or Nursing Mom's Tea by Yogi Tea (both of which can be found at helath food stores), which I would drink entirely before noon. The teas have fenugreek and other herbs that are supposed to increase milk supply. I'm still not sure if it was the herbal stimulus or just the fact that I was drinking tons of water/tea, but I swear that the tea helped me (probably the latter and a bit of placebo effect!). In fact, I would buy boxes and boxes of either tea at a time, because I was terrified that I might run out! FYI: I usually brewed the tea the night before (4 cups), and then after it cooled I would transfer it to a large reusuable water bottle so I could drink it during my morning with ease.
Other bits of helpful advice:
1) Don't compromise on your 2 earliest pump sessions of the day; morning is when prolactin is highest (and therefore milk production is the highest) so make sure you have adequate time to pump during that time. I always pumped before rounds started (like at 8am), and then again sometime between 11:30am and noon (I was flexible about my 3rd session, which would be somewhere between 3:30 and 5pm). I had to adjust my schedule and get to work a bit earlier so I had time to pre-round and then pump before rounds, buy starting my day with at least one great pump session helped reduce my stress level, so it was worth it!
2) Try to eat a lot of grains, especially oatmeal, including oatmeal cookies! Grains are supposed to help with supply, plus it's healthy for you! By the way, this includes beer, so feel free to treat yourself to a small glass at night to help you relax and boost your milk production at the same time.
3) Make sure you have enough healthy fats and good protein in your diet. Sometimes we restrict our diets too much after childbirth in an attempt to lose the "baby weight." However, if you limit yourself too much, it can affect your milk production (this is especially true if you are a busy intern with no time to eat!). On the days when I was producing less than normal I would often have ice cream (which is fat and protein ) or pizza, and I swear that the following day my milk supply would be back to my normal amount (again, probably placebo effect and not scientific in the least, but it's worth a try!)!
4) Remember that every woman is different. On my best pumping days I produced 17 oz/day pumping 3 to 4x/day. That only lasted for 3 months, after which I could pump about 12 oz/day. However, I had a friend who could pump 10oz in one session!!!!!!!!!!!!! One time I saw her gigantic freezer stash with its HUGE bags of milk and I was so jealous (ridiculous, I know). But you know what? Everyday I was usually able to pump enough for my baby, and I was always had enough milk to BF her when I got home.
5) Try nursing your baby on demand while at home, and consider co-sleeping so you can nurse during the night. I know it's not for everyone, but co-sleepin at night let me get enough sleep and it helped keep my supply up.
Here is what my pediatritian told me when I had the same problem:
when babies want to increase your supply, they suck faster and harder when they first latch on. So, try to pump at a lower, slower setting most of the time, then when you need to increase the supply, kick it up for about 3 minutes, then go back down to the medium setting. This really worked for me. Before that, I had been pumping on the highest setting everytime since it was faster. Keep it up and good luck! I swear breastfeeding is harder that being pregnant.