Robyn, We had a baby doing that not too long ago.  Rather than taking the
baby off each time (that was frustrating for mother and baby) I suggested the
mother first try to pull baby's lip out (or have helper do it), then pull
baby in real close and tell the baby (in a positive tone of voice) what a
good job she was doing and how well she was nursing.  Sounds simple, I know,
but it worked for that particular mother/baby pair.  It was important for
mother to hold baby close, too.  When mother relaxed her arm and baby slipped
down the bottom lip went right back in.  Pillows to help support mother's arm
and close attention to baby's positioning worked out well in this case.  Hope
it helps.  Keitha :)