I struggled with keeping my supply up while working as well. I can't say that any of these things worked in particular, but I was able to pump enough until our son was about 10 or 11 months and then added formula for daycare.
-Fenugreek or mother's milk tea
-Drinking water when thirsty, not too much. What I did was only drank water, or milk (no soda, junk drinks)
-Set my alarm to pump my breasts in the middle of the night (midnight or so) to add another pumping session to my already 3-4 times at work.
-Eat oatmeal for breakfast and whenever hungry at work
-Nurse as much as possible
-There are drops that I took several times per day that had fenugreek and blessed thistle in them. I can't remember the maker, but I purchased them online. It tasted horrible, but you only need one or two drops in your mouth or drink at a time. (you'll smell like maple syrup from this and the tea)
-Eat healthily and heartily
Good luck!
P.S. The drops/tincture that I mentioned above are from www.motherlove.com (like another mom indicated above)