1) Never mail a plant in an envelope and only mail seeds if the envalope is padded.
2) Wrap WET papertowels or long-fibered sphagnum around the plant and its roots
3) Priority is best but not nessicary, First class is fine.
4) Use only priority when it is really hot or cold outside (IE summer and Winter)
5) To wet sphagum peat moss or long-fibered sphagnum pour boiling water over it. This is recommended if you have a rubbermaid container or some other container that will not melt/crack. This wets sphagnum instantly
5) If a trade seems to good to be true (usually a lot of plants for one or two of your location/rare/prized plants) check the person out. Ask others (moderators, reliables) for info on that person. It might be a scam.
Thats all I have. If this could be sticky-ed and added too every time someone messes up we can insure it will not (hopefully) happen again.