SPECIAL NOTE: Just to get this out of the way up front: there are any number of perfectly valid ways to send plants through the mail. I am describing a way that works and is convenient for me, but it's not the only way, not by a long shot, and persons who have mailed me plants in the past should not interpret the instructions which follow as an indication that they did a bad job packing.
First, you will need to try to determine whether you even can send plants through the mail. If you're not a business, you probably technically can send anything wherever, but some U.S. states ban the import of plant material and/or soil from certain other states. Shipping citrus fruit to California, for example, is a no-no. I've located California's list (it's the only one I've needed to find so far), but some of the other western states have similar restrictions.
For what I'd hope would be obvious reasons, sending plants known to be invasive in the state you're sending them to (i.e. mailing Ardisia elliptica to Florida), or plants which are diseased or infested with insects or other pests are bad ideas also.
I asked the Postmaster here about mailing stuff into states with quarantines, and she said she honestly didn't know what happened in those cases: she was sure they would run packages by drug- or other-contraband-sniffing dogs at some point, and maybe also x-ray them, but unless there was reason to think that there were drugs, banned plant products, or other contraband in the packages, she didn't think they were ever opened. So it's quite possible that you could send kumquats to California and get them in anyway. Which makes it all the more important that you try to find out what would be harmful: these restrictions are in place for a reason. If you send a diseased kumquat to Los Angeles and it destroys their whole citrus crop, guess who's paying more for orange juice at the supermarket.
And I guess probably also you, but I'm less concerned about that.)
Other countries are a much more complicated situation, and I'm not clear at this moment whether it's possible for me to, say, ship a couple houseplants to someone in Canada or Mexico. (I'm pretty sure anywhere else in the world is out of the question, if for no other reason than it would take a really long time to get there and/or be prohibitively expensive.) I'm working on finding out, especially with regard to Canada, but if anybody happens to know the answer already, or where I could find the answer, please leave a comment.
In the U.S., they'll ask at the post office if what you're mailing is perishable. (I'd hope that other services like UPS and FedEx do this as well, but I don't know.) I always say yes, and that it's plants. I'm not sure what this actually means in terms of how the box is treated within the postal system, whether your box is actually kept somewhere with moderate temperatures or just gets thrown in with everything else anyway. So if it's going to be very cold along the route, or if the plant may have to sit outside in the sun for a few hours on a hot day, you're probably best to wait and send it later. I usually (always?) send stuff between April to October. Cold is more of a problem for most tropical plants (i.e., houseplants) than heat is, but extremes of either can be fatal, so think first.
Then you'll need a box. For small items, like a single Kalanchoe leaf, you could use a padded envelope, but even for small things I prefer the added strength of a box. I don't know what sort of machinery envelopes get put through.
If the recipient already knows what they're going to be getting, then you don't really need to include a list, or ID tags, but it's neighborly to do so anyway.
Usually I don't: instead I slice up an index card and tape it to the plants after they're wrapped up, but I'm getting ahead of myself here. For our first example, I actually did write a tag (piece of a plastic gallon milk jug: Mr. Brown Thumb is responsible for that idea) and stuck it in with the plant, a Hatiora salicornioides:
Next, I line the bottom of the box with some packing material. In the past, I've used shredded paper, crumpled paper, styrofoam peanuts, and plastic bags, to various degrees of success. I think I prefer plastic bags, now that I've tried them all a few times, but the particular material isn't that important. The idea is just to have a layer of something that will stay between your plant and the edge of the box. This will protect it somewhat against temperature swings, as well as some kinds of mechanical damage.
Then, I put the plant in one of the plastic bags,
loosely tie the top of the bag onto the plant with a rubber band (this keeps the soil from leaking out all over the inside of the box, and also slightly reinforces the stems against crushing forces against the top of the plant),
and then put all that into a second bag and rubber-band it far away from any plant parts. The second bag is not strictly necessary, but in this particular case I wanted to make doubly sure the soil was contained. Some mail-order sellers will insert two bamboo stakes into the soil, to hold the bag up away from the plant and protect against crushing forces against the top of the plant. It shouldn't be necessary in most cases.
Some people, instead of bagging the plant, put packing tape over the top of the soil, or fold aluminum foil up around the edges of the pot, to hold the soil in. Those both work, though I don't do them because: tape is hard to get back off, afterward, and sometimes it sticks to the plant or the packing material and has to be peeled away carefully, and aluminum foil runs a slight risk of injuring the stem.
If they're all you've got to work with, then by all means use tape and/or foil, but try to avoid it if you can. Also I should probably note here that professional plant-shippers tend to stick a wad of shredded paper around the top of the soil and then use packing tape to tape it down to the pot. Again, sometimes getting the tape back off again can be a problem, but it is probably the best way of keeping the soil in place, if that's a major concern.
Anyway. Then the bag gets set down on top of the cushioning layer of stuff you put down first. If you're going to attach a piece of index card with an ID, now's the time to do that.
For plants that aren't already potted-up, like these Cissus quadrangularis cuttings,
the process is basically the same, though I used a sandwich bag to first wrap some soil around the roots before bagging the whole plant up.
Again, tying a rubber band on -- not so tightly that you hurt any stems, but tight enough that the soil's not going to shake its way loose -- can be helpful. Not reliable on its own, but helpful.
I didn't trust the rubber band to be that great in this case, though, so I put the baggie and plant on the side of another plastic grocery bag --
-- and then rolled the bag up around it.
Fold over the loose end with no plant or soil in it, and tie it down with a rubber band like in the first example, and then it's done too and can be set on the pile.
Try not to put plants directly on top of one another if you can help it: you don't want a carefully-packed fragile plant to have a heavy pot resting directly on top of it, and since the box is going to get spun around during shipping, you don't know what direction "on top" will be at any particular moment. For this box, I did try to do a sort of lasagna-packing approach (layer of bags, layer of plants, layer of bags, layer of plants), but I don't usually if I'm working with a small box or limited number of plants; I just try to jigsaw them all in there together in a way where they can't move around much.
You may be wondering about watering. Generally speaking, if the plant is okay when you put it in the box, it's going to be okay when it comes out of the box too: it's dark in there, so it's not going to lose a lot of water to evaporation, and transpiration is reduced simply because there are so many layers of plastic standing between your plants and any dry air that might be outside the box.
Another good reason not to water before mailing stuff, unless you really have to, is that the price to mail your box will depend partly on weight, and wet soil weighs a lot more than dry. (With the U.S. Postal Service, the destination zip code and dimensions of the box also factor into the cost, FYI.) So it costs you to water first, too. If the plant really is dry, you can also just dribble a little bit of water in and then seal it all up. It's not a good way to water in general, but these are exceptional circumstances.
I should possibly take this moment to mention also that a lot of plants like Sedum morganianum, Echeveria spp., Pachyphytum spp., Crassula ovata (jade plants), Saintpaulia cvv., Begonias, etc., are likely to break apart to some degree during shipping even if you're very careful about how you do it. Generally, the plants that tend to shatter also tend to be pretty easy to start from the pieces, so it's not a huge loss, but you should warn the recipient in advance that this will probably happen, because otherwise they may open the box and see a shattered Sedum in there and think it's dead, or be angry with you, or whatever.
As a final example, we have some Plectranthus 'Mona Lavender' cuttings. These were just cut, so they'd be as fresh as possible, but I still had to do something so they could ship without drying out.
So I took a paper towel, and laid it underneath the bare stems (from which I'd removed side shoots and leaves),
folded the bottom of the paper towel up over the stems,
and then rolled the whole thing up.
Then I got the paper towel damp but not sopping wet,
and proceeded as with the Cissus example, putting the plant into a baggie and then into a plastic grocery bag. (I think. I may have skipped the baggie. Two layers would have been unnecessary, whatever I actually did.)
When all your plants are in the box, you should still have a small layer of space available to put packing material on the top, bags or whatever, before closing up the box. I use packing tape to close up the top, and then often put packing tape along all the edges of the box too. This is mostly because I'm neurotic and enjoy taping things, but some of it is also so that if I've done a bad job and soil does start rattling around inside the box, at least it's not going to fall out through the edges and seams and leave trails around the Post Office.
If you've packed the box well, you should be able to pick it up and spin it around without hearing or feeling plants shifting around a lot inside. If you do hear or feel this, you probably should re-open the box and add more bags (or whatever you're using to pack); every shift is a chance for damage to happen, and your box is going to get thrown around a lot before reaching its destination. Too much packing material, on the other hand, is just going to push against the plants and add to the shipping weight, so try to add in just what you need to keep it from shifting around, no more.
Then you can address your box and take it to the post office (or wherever).
I personally try to send all my plants out on Monday or Tuesday, so as to minimize their chances of sitting unclaimed in a closed post office on a Sunday. It also seems to help them get to their destination faster if they're sent first thing in the morning, though that's a lot harder for me to do than the Monday-Tuesday thing, and may only be true for Iowa post offices or something.
Boxes sent by Priority Mail (first class), through the USPS, generally take three or four days to arrive, though I've had a couple arrive in two. Third-class mail is significantly cheaper, but it's also slower (five to ten days), so I don't recommend it unless you're sending exceptionally sturdy plants that won't mind being in the dark for a week and a half.
You should instruct your recipient (if they don't already know) to open the box and let the plants breathe as soon as possible on arrival. It's not necessarily an emergency if they don't pot up all the cuttings and stuff right away, but staying longer in the box without light or fresh air isn't probably going to do them any good, either.
So that's my advice. Contrary opinions or experiences, questions, or comments are welcome. Also this was written quickly and without a lot of proofreading, so if you catch any redundancies, typos, misdirected links, etc., I'd take it as a kindness if you pointed them out.