It's really common when they're small to fall asleep on the breast! You want to watch for the sucking to go from the slow suck-swallow of drinking to the short butterfly suck-suck-suck of comfort sucking. Breast compression can really help to increase the flow and wake your lo up (check the site that maggieruth kindly provided!). I would also change my baby's diaper halfway through to wake her up and get her to eat more.
Once she gets a bit older, she'll start staying awake more and you can worry then about feeding to sleep! :wink: