You all probably know these tips already, but in case you don't...

Old cotton T-shirts work well for clean, dry shells when you have no other
packing material and not much space left in the luggage. Remember to
reserve one shirt to wear home on the plane.

My mother used to send me fragile items surrounded by plastic bags filled
with marshmallows, which puts a new meaning on "eating your fill of
marshmallows." Never had any problem with them.

Sand dollars can be packed in pairs, flat oral surface to oral surface,
with only a little padding between them.

If time is short (as when night is falling), small shells can be placed on
a sheet of packing material and then rolled up. Aluminum foil works very
well for this; the crinkles in the foil act as a shock absorber. Shells
that do break are immobilized, so the fragments can be reunited. (I've only
tried this in land or freshwater situations. Does aluminum foil react with
seawater?)

Glass is great for long-term storage, but one of the worst materials for
transport. I'm surprised that no one has mentioned bubble wrap or plastic
"peanuts" yet; perhaps they're too obvious.

Fossils can be packed in sand or mud from the same layer that they came
from, usually without harm as long as the fossils are separated and the
mixture is packed too tight to move inside the bag or box. Sediment is
heavy, but it costs nothing and generally contains additional small shells.
Dry leaf litter can also be used, but often contains insects and is best
removed immediately on your return to the workroom.

Cigar boxes have the ideal depth for collecting fossil shells without
crushing them. The traditional packing material is cotton. In practice, I
don't smoke and cotton is expensive and a little sticky, so I use somewhat
deeper boxes and an assortment of plastic bags, aluminum foil, and sediment
as appropriate.

Andrew K. Rindsberg
Geological Survey of Alabama