i use same techniques described by other posters, but i'm bored and want to post too..
- un-pot plant and shake off most of the soil, careful not to damage roots.
- wrap roots in damp (not wet) paper towel, (i splash drops of water after wrapping dry towel)
- saran wrap the towel covered roots, (i place plant in middle of saran wrap, and fold over, and over)
- saran wrap the whole plant, (it usually needs a second saran sheet to cover the top of plant)
and so ... i used to wrap plants in a plastic bag to keep it from moving around in the box,
but now i usually just tape the saran-wrapped plant down in the box so it doesn't move.
i like to use painters tape so that doesn't make it impossible to remove from saran wrap.