The main thing to keep in mind, is that when you are at work, you are pumping the milk that the baby will eat the next day. So on Monday, you are pumping for Tuesday; on Tuesday you are pumping for Wednesday, etc. You should pump at the office at the same times your baby typically eats.
Before you go back to work, you should get accustomed to using your pump. Try pumping after the first feeding in the morning, when your supply is the highest. Put this milk into the freezer and save it for emergencies... spilled milk, traffic jams... or even date nights with your spouse. To further increase this back-up supply, you might try pumping after other feedings as well. Some moms are even able to pump from one side while feeding the baby on the other. But keep in mind that you can never pump as much as your baby can suck out.