I'm by no means a veteran, but people who have sent me pins have done these things:
- Sometimes put each individual pin in a small plastic ziploc baggie (the tiny ones, not the regular sandwich ones)
- ALWAYS wrapped them in bubble wrap; sometimes each pin wrapped individually, and sometimes two or three pins rolled in a larger piece, but ALWAYS so that no pin touches any other pin (so they can't rub against each other and scratch) - some people have pinned them to the bubble wrap so they can't slip out and others have just taped the wrap shut
- Sometimes pinned them to a square of thin flexible foam rubber backing if they didn't have a card
- ALWAYS sent in a bubble mailer
- always sent with delivery confirmation (not everyone has sent me the DC number, but I've gotten the impression that it is best to do so; then the recipient can track the package and let you know if anything goes wrong)
And some people make it extra special by including a postcard or a handwritten note or stickers or whatever cute thing takes their fancy. I'm pretty sure that's not expected, but it does make me grin every time, and so I started doing it too.
Also the biggest thing is communication. Once you agree on a trade or a sale, tell the person what day you plan to send it; if something comes up and you can't do it then after all (you get sick, you forget to bring them to work, whatever), contact them again and let them know what happened so they're not left hanging, and let them know when the new send date will be. Then let them know when you actually do send it, and give them the DC#.
That's what I've gathered from reading posts here and receiving pins myself. I hope people will add anything I've forgotten.