Try sleep-sharing, also. My baby girl does a LOT of nursing at night to sort of recharge her mommy battery at night and it helps keep my supply up.
At work, I drink Mother's Milk Tea by Traditional Medicinals. It contains Fenugreek which is an herb and natural supplement. (Pill forms of fenugreek can be found at most health foods stores like GNC) Also, I have a picture of my baby, a quiet place to go pump, and I try to imagine hearing her cute little baby noises. Somewhat of a meditation on baby, make it your own little ritual. I pump every two hours. Don't wait until you feel "Full" to pump. Keep your cycle going. I also pump once in the morning before we head out the door so she has fresh squeezed mommy juice first thing.
On weekends, if he's sleeping, pump, if he's happily playing somewhere, pump. (Use those memories during the work week to relax and help yourself let down.) Feed him on demand and let him sleep in your arms.
Yes, it is hard work to maintain a breastfeeding relationship while working, but it can be done. And it is so worth the benefits for both mom and baby to keep it going. Good Luck. And remember, it's all about supply and demand.