If you're just sending several strips of film in their original plastic or paper sleeves, we recomend cutting two pieces of cardbord (from a box for example) that's only slightly bigger than than the strips themselves. Then sandwich your film between the two layers of cardboard and tape around the cardboard taking care not to get tape on the film. As an extra precaution, place the taped cardboard and film into a Ziplock type bag to protect against moisture or water damage during shipping. Then place it all in a shipping envelope or small box.
If you're sending a larger shipment, make sure you leave the film in their original plastic or paper sleeves. Then place those sleeves into a Ziplock type bag to protect against moisture or water damage during shipping. Place the Ziplock type bag into a sturdy box.
Don't wrap your negatives or slides in paper or other materials that may attract dust, particles or paper pulp. Make sure to keep them in their original box or carousel if possible.