When you agree on a trade and receive the other person's address, write it immediately on a mailing label. Write what you are to send on the back of the label to avoid confusion.
Some have suggested keeping track of trades in a notebook, in case either trader would have computer problems.

Plan to ship on Monday or Tuesday so the package doesn't get stuck in the Post Office on the weekend. Priority mail takes three days, and costs around $3 (subject to change, of course!) for a two-lb package.

Either dig the plant the day before you ship it and soak the roots in water overnight,or leave it in the soil until you are ready to pack it, but thoroughly soak the soil around
it the day before.

Try to include plenty of roots on the plants that you send. (very important!)

Gently rinse most of the soil from the roots.Wrap the roots in a moist paper towel, wet but not dripping.

Put the roots and the damp paper towel (but not the leaves) into a plastic bag and wrap the bag snugly around the stems with a bag tie, to seal the water in. Be careful not to crush the stems!

This is a step that not everyone does but it seems to help: Lay the plant (with the roots in the baggie) diagonally on a sheet of news paper and roll the paper around the whole plant gently so the foliage is protected by a cone of newspaper.

Label the plants with water proof marker.

Box the plants in a priority mail box. (The Post Office supplies these boxes and nice labels free upon request.)

Pad the plants with lightly crumpled newspaper so they don't knock around in the box.

Write "LIVE PLANTS" on the label.

Keep the package cool until you mail it. Don't let it sit in the sun in your car! It's a good idea to notify the recipient once you've mailed the package, and ask them to let you know if it arrived safely, and the condition of the plants.