A month prior to your return to work, begin to build a bank of breast milk in your freezer. You'll be able to leave on your first day back with a clear mind that you already have a significant stash of milk for your baby. At work, you will be pumping at the same time your baby would typically feed. If you're a salaried employee, you're probably a bit more flexible than if you're hourly, but the process is still the same. You just need to steal away for 15 minutes per pumping session. Take a deep breath, relax and put on some music. Put your expressed milk in your cooler, and bring it home to add to the bank.
You can either rent or buy a breast pump.
You should speak with your employer prior to your return to work. Explain that you are breastfeeding and plan to pump during the workday. If you do not have a private office where you can close the door, ask if there is a conference room or another space that isn't being used. You should not be expected to sit on a toilet seat in the bathroom to pump your breast milk. The experience should be relaxing. Some women have even found that pumping in their car has been the ideal location.
FYI: Check out the House Resolution 2236: Breastfeeding Promotion Act of 2007. It was introduced by Rep. Carolyn Maloney [D-NY].