Is is possible to keep up a good milk supply and work outside the home? The answer is, yes! I am currently breastfeeding my 3rd baby, and have worked part time outside the home with all 3 babies. My youngest is 6 months old now.
There are a few important things to remember: breastfeeding works on supply and demand. The more baby nurses the more milk you will have. Also the reverse is true: the less baby nurses, the less milk you will have. In order to be successful, you should not supplement with formula. Every feeding that you use formula is one less time that baby is nursing, and therefore will decrease your supply of milk. Before going back to work, pump at home to store up a supply of milk in your freezer. Date the bottles with a small sticker, so you can use the oldest milk first.
Once you are at work, you should try to pump as often as baby is feeding, usually every 3 hours. Again, the principles of supply and demand apply. If you are not pumping as often as baby would be nursing, you are decreasing your supply. It is also very helpful to nurse on demand at home and not put baby on a strict feeding schedule. Babies use "cluster feeding" to increase your milk supply, especially if they are having a growth spurt. It is best to follow baby's cues and nurse when he or she wants to nurse. Your baby will also learn to self regulate how much he or she eats.
Like I said, I'm nursing my third baby and we have never had to use formula, not once!