Here are some tips that worked for me:
1. Make sure that you have the correct flange size. If they aren't they can pinch milk ducts and prevent your breasts from emptying. This reduces supply because breastmilk is supply and demand- the more often and more fully they are emptied, the more milk your body thinks you need.
2. Make sure you pump 8-12 times a day, including through the night. This is very hard at first but essential due to supply and demand. I pumped every 3.5 hours at night and every 2 during the day at first. As my supply established I gradually increased the time at night until I had two 4.5 hour stretches at night around 10 weeks postpartum. At 8 weeks your supply is regulated by demand entirely so you can attempt longer stretches starting then. It takes 24-48 hours for your actions to cause a change in supply, so I always tracked the time I pumped and oz. This way, I could go back and see what caused changes and correct it if needed.
3. Use a cover when pumping. This helps you to relax and not focus on what's coming out.
4. Relax. Look at pictures of baby, read a book, watch a TV show, play around online. At night I listened to relaxing music and looked at pictures of her and I noticed a significant different in output if I didn't do this.
5. Drink tons of water. You need between 64 ounces and half your body weight in ounces per day anyway. This means that if you weight 140 lbs, you need 70 oz of water per day. Then add about 25 ounces for breastfeeding (your body gives out about that much water every day to produce milk). I had a large cup of water with me and aimed to finish it every time I pumped.
6. Eat oatmeal every day, but not the instant kind. Quick oats are fine, but I noticed when I had the time to cook steel-cut oats I had the biggest increase. I typically avoid oatmeal because it has so many carbohydrates, but I found that I needed it while breastfeeding.
8. Get enough calories. Sometimes it's hard to find time to eat, but if you don't get enough nutrition your body will struggle. I did this by having a healthy snack every time I pumped, like apple slices and almond butter. At night, almonds and dried fruit were my best friends.
7. Drink lactation tea, and tons of it. You can buy it in the bags, but I got it loose-leaf and drank about 8 cups of it a day. This counted toward my water consumption since otherwise I'd probably be swimming. I actually made it 4 cups at a time in my french press and sweetened it with liquid stevia.
8. Take motherlove more milk plus supplements. They are made from herbs and have no additives or filler and WORK! They cost a lot but are well worth it if you struggle with supply.
9. Massage while you pump. I would massage my breasts toward the flange and squeeze, especially places that felt hard. Many women find they can only empty their breasts while pumping with this method.
10. Play with the pump speed and suction. Higher speed initiated letdown and can be used several times throughout a pumping session. If your pump lets you alter speed, try different ones to see what works for you. If you can stand higher suction, it can pull the milk out more efficiently to a point. If you are uncomfortable, your body won't let the milk down. If the speed is too high, it can actually close off the milk ducts and trap milk.
11. Don't stress. Easier said than done, but so important. I would stress over making less milk at certain times of day or getting less one day than I had the day before. If supply continues to dip you should look into what could be causing it, but temporary changes happen all the time. One day I would get a ounce or two less than normal and another I'd get three more ounces than normal. One day my afternoon pump would be terrible while the next it would be great.
12. REST! This is so important. If you can nap during the day, do it. If not, lay down and relax.