When we get a baby who tucks in loser lip, we try having mom bring baby's
bottom up under opposite breast using a firm foam pillow (so it doesn't
collapse during the nursing session) lift and support breast with the side of
the hand or use a sling, touch the baby's chin to the breast and use the
lower areola/breast to roll down the lower lip right before "sneaking in" the
nipple. Mom then quickly pulls in on baby's shoulders to keep the chin
pressed in tight.  If the lower lip rolls in, mom or helper pulls out the
lower lip and pulls baby in tighter.  We try NOT to take the baby off the
breast too much unless mom's nipple starts hurting because everybody gets too
frustrated.  If mom puts her feet up on a small stool, the lap helps to
support the pillow which keeps the baby supported.  Also use a folded towel
or blanket to wedge in mom's wrist to keep baby supported at breast level.
 When moms are holding babies in tightly, their muscles can get tired so firm
support that doesn't collapse is important in maintaining a position.  I
often find that after a few days to a week, everything gets easier.