Suckling at the breast.
Besides a let-down reflex, the baby suckling at the breast stimulates the production of the hormone 'prolactin'. Prolactin sends a hormonal message to the woman's body to produce more milk. When the baby starts to suckle on the breast, prolactin levels start to rise dramatically. (They also rise to a lesser degree when expressing milk.) The amount released is directly related to how long the baby sucks for. The prolactin level peaks about 30 minutes after a feed and returns to its previous level after 3 to 4 hours (unless the baby feeds again before this time).
The more the baby feeds, the more prolactin is released, and the more milk the woman produces in the 12 to 24 hours following (even if her breasts 'feel empty' because the baby is feeding frequently). Prolactin release is also related to the woman's circadian rhythm (or her body's biorhythms in a 24 hour period). Prolactin is released at higher levels during evening and night feeds, and at lower levels during morning and day feeds.
The general rule of thumb with breast milk production is:
Supply = Demand
Or, the more milk that the baby removes from the breasts, the more the woman's body will make in the following hours. Your breasts are never really 'empty'. This is because they are constantly producing milk, and if you are feeding frequently, they are constantly getting the message to 'make more milk.' Some women think about their breasts as being more like a factory, (continually producing milk to meet increased demand), rather than as vessels or containers that 'empty out' when the baby feeds. Unrestricted feeding on demand is the key to a good milk supply.