If you plan to go back to work, starting pumping at least a few weeks before you go back. This will help you get use to pumping and will also give you a small stash. I starting pumping when my daugther was about 6 weeks old and pumped twice during the day and then during the night (she is a great sleeper, so I needed to empty them!). This allowed me to build up stash, which has come in handy many times.
When pumping at work, you need to keep a regular schedule, keep hydrated and eat enough to maintain your supply. The days I get off schedule or when I've gotten sick, I pump less and I'm glad I have that stash at home.