I lost a lot of blood after a c-section and my milk wasn't coming in. When it finally did, it was a pretty meager supply. I did some research online and talked to a few lactation consultants and here's what worked for me...
I drank Mother's Tea, but can't really tell you if it helped or not. What did seem to help was 1) drinking lots of water (I drank about 120-130 oz a day), 2) regular pumping sessions (pumping at the same time every day, not missing sessions), and 3) regularly throwing in some extra 10 minute sessions (5-10 minutes after you nurse your baby or pump) until my supply went up. I read somewhere that it's not so much how long you pump but how often that increases your supply.
For a while, I did feel like all I did was pump/nurse, but it did get my supply up and I kept up with my baby for a year. It's a big committment and not for everyone. One lactation consultant told me to give it a shot for as long as I felt like it and then not feel guilty about my decision to keep nursing or switch to formula. Good luck!