When do you express? How old is your DD? I'm guessing at least four months old, so nursing and expressing at the same time will take a bit of practice - try holding her in the rugby hold (I sit nine-month-old DS up and he latches himself on and taps the pump as I express!) and express from the other side - makes the letdown more powerful.
Other than that, try expressing before the first feed of the day or if your DD goes to bed and stays asleep a while at night, wait a few hours from the last feed and express then.
Try to build up a supply of milk in the freezer for when you go back - I was too blase about it and have been running to keep up some weeks. Initially, DS wasn't eating much food and needed 20oz when I was doing a 10-8 shift, plus boob in the morning and sometimes that night.
But now he's eating more, he's taking less milk during the day - I left 15oz today and I don't expect he'll drink it all.
I never leave more than 4oz in a bag as it's easier to make bottles up with 2 and 3oz bits - sometimes he'll only take 2oz, sometimes he'll have 3 or 4 (he was taking 7 happily (once he took 8) as his last feed when I first went back to work when he was 6.5 months). It's MUCH better to defrost another couple of ounces rather than have three poured away because you've frozen too much in one bag.
You'll soon work out how much your DD will take. To get her used to a bottle though, don't use more than 1.5-2 oz!
Before you express, massage all round your boob, then lean forwards and shake them (looks weird, but helps, I promise!). When you pump, hold the handle in until the milk stops flowing on each pump - you'll need to vary from the short, quick 'sucks' to the longer ones as the flow varies. When it dwindles, swap boobs, but do this a couple of times as you might get a better letdown the second or third time.
HTH - if I think of anything else, I'll post again