Q&A: Maintaining Milk Supply?
First, keep in mind that baby can continue to receive your milk while you are at work. (You'll just need the help of a good quality electric pump.) If your employer isn't excited about allowing you to pump on the job, talk with him about the benefits (like a baby that is sick less often, meaning less time off for you). In some states, employers are required by law to allow you to pump breast milk for your baby.
If you still decide to wean baby from the breast during the day (or wean yourself from the pump), do so slowly, first eliminating one feeding/pumping, and then waiting a few days for your body to adjust before eliminating another. (If your breasts become engorged, express a small amount of milk.) Your body has an amazing ability to adjust your milk supply to meet your new breastfeeding schedule. You shouldn't need to do anything to maintain your supply for evening nursing, as long as you continue to consistently nurse at night. If you begin to have problems producing enough milk for evening feedings, talk to a local lactation consultant (IBCLC) about your specific situation and concerns.