agree with mapo0kw, if the baby doesn't latch on correctly, nursing will not last for long
pain can be a guide for us to know whether baby is on the breast the right way
The goal is to get the baby to compress the areola (the dark area around the nipple) to get milk, not to nurse from the nipple
The nipple needs to be in the back of baby's mouth during nursing in order for him to correctly compress the areola and activate milk flow
check the positioning by looking to see whether most of the areola is in baby's mouth
the correct positioning is fairly tight
If nursing hurts, chances are the baby is latching on to the nipple.
Remove the baby and reposition
Gently insert finger into the baby's mouth to break the baby's grip on the breast and begin again.

source: health.yahoo.com/.../healthwise/popup/tx1042
