J.--
First of all, good for you! Itis definitely harder pumping when returning to work.
What seemed to work for me was taking Fenugreek caplets (helth food stores) and drinking Mother's Milk tea (also health food stores). I think if I recall, curry is also supposed to help supply, but it's been 4 years now so mymemory is rusty!
With my first daughter, I did have them keep a sleeve of formula (powder) on hand for that last bottle of the day if need be. The rarely used it, and were super great about trying to get my daughter to hold off until I arrived and then I would bf her before driving home. You might want to try pumping late at night too to get just a few ounces extra that you've missed gathering during the day.
I pumped 3 times a day at work, and usually got 2-3 ounces at a time...though there were days I'd get four each sitting!
good luck!
J.