Help! Loosing my milk supply!

I've been exclusively pumping and have had a good breast milk supply until going back to work. I use to be able to pump 5oz out of one boob and 2-3 out of the other. Since returning back to work the amount I pump at one time has really gone down. I get so busy at work it gets hard to pump as much as I should. The past 2-3 days I've only pumped twice in a 9hour period at work. Is there a way to get my supply back? Do lactation cookies really work? I would like to keep pumping at least another couple of months.