This is what I do...
Get a sandwich bag (ziplock) or other plastic bag, it doesn't need to have a seal but is best if it does I suppose.
Take the plant/s out the tank and let the some of the water run of, to leave the plant wet, so you don't get a load of water leaking in the post.
Put the plant/s straight in the bag and seal, if you havn't got bags with a seal tightly secure an elastic band.
Then using an envelope with bubblerap attached to the sides place the sandwich bag with the plant in it, if you tilt it one way there will should be a cm sqaure of water in a corner, maybe more.
Or if you do not have one of these protective envelopes just wrap the sandwich bag in a bit of bubblerap to protect the plant from getting squished/bent.
And the feedback has been great, saying that they were in top cond on arrival.