My own body began weaning my baby at 3 months, unfortunately, and so I'm afraid I am not personally much help. However, I can tell you the things people recommended for me, and they kept me producing some milk until my son was 6 months, although it didn't work indefinitely.
The first is to pump and pump and pump. Pump after feedings, but also pump every 45 minutes, if you can. Tell your body you need more!
Secondly, check out Fenugreek. It's an herbal supplement that is supposed to increase breast-milk. Read the instructions on the packet to make sure you are taking enough of it, and give it at least 2 weeks to kick in. Products like Mother's Milk tea are also supposed to be good.
Begin eating good-fatty foods, and make sure you are drinking A LOT of water. Make sure you're drinking at least 2% milk, or make fruit milkshakes, eat avocados, unsalted nuts, and all the other good fats. Be careful with this one - it is effective, but I also gained back almost all of my baby weight in the 3 months I was on the diet, and the weight has been a lot harder to lose the second time around.
Cut down on anything you eat that is dehydrating. That includes anything very high in sodium (chips, salted nuts, Chinese food - you name it) as well as sodas and other carbonated drinks.
I hope that you will be successful in increasing your milk flow. However, worst-case scenario, talk to your doctor about how to supplement your child's diet with formula without losing more milk supply. By the time my son was 6 months, I had been struggling with supply for over 3 months, and my son would have starved eating just my milk. But as my doctor kept saying, every drop is liquid gold. Even if you cannot feed exclusively breastmilk, keep the breastmilk coming in whatever portions you can to give your child the full benefit. Each drop is better than none at all. Good luck.