Cardboard box, a label, some peanuts... and a whole bunch o' tape!

Anyway, stave off the inevitable by packaging your equipment to prepare for it.
Don't try and stick everything into one small box; get the next size up, and stuff the spare room with styrofoam peanuts, bubble-wrap or plain ol' wadded newspapers.

For example: You're mailing in a shiny new AutoBlastemFlatter 6000 to have the magnetic containment chamber realigned and the neutron accellerator degaussed.
Take the barrel off, and wrap it seperately in a couple of full sheets of newspaper. Don't wind 'em tightly, you want some 'cushion'. Take the tank off, and if you're sending it along, wrap it in bubble-wrap if possible, or several sheets of newspaper at least. Especially the fiber-wrapped tanks; you want to make sure the wrapping doesn't get damaged. Remember, if you're shipping an air or CO2 tank, it MUST be empty! No exceptions!

Again, if possible, wrap the gun itself in the big-bubble wrap, or the thin foam sheets, or, if nothing else is handy, the tried & true newspaper. If you use newspaper, again, don't wrap it tightly, and, it's not a bad idea to "prewrap" fragile items like guages. To prewrap, just take a small section of the paper and scrunch it around the part, so that piece has a little extra pad.
It can't hurt to tape the paper on each individual "package", so it doesn't slide around and expose something to get scratched. Remember, that box will be tossed, turned, transported upside-down, stacked underneath a box of anvils, and generally slammed around.

If you have some small parts that go with it, too small to wrap individually (for example, you've taken the trigger assembly apart and can't get it back together) put them in a zip-lock baggie, and if possible, roll that baggie up and put it in *another* baggie.

Now then, with a handful of individually wrapped bundles, it's time to put 'em in the box.
First, lay down some styrofoam peanuts, bubble-wrap or some wadded newspapers at the bottom of the box. Next, lay the heaviest item in. Not necessarily the biggest, the heaviest. If you can fit the other pieces alongside it, do so, but leave room to pack peanuts or paper in between each bundle.
If necessary, pour another layer of peanuts or papers in, and put the next 'layer' of parts in on top. Again, as you put things in, leave room to wad padding in between each item to add to the cushion.

Once all the items are in and packed, double-check that you do, in fact, have *everything*. Personally, I constantly forget things and have to cut the tape back open to add that one item.
It's a very good idea, no matter who you're sending the box to, to include a note inside with your name and return address, so if, for some reason, the package is lost or misrouted, you have a better chance of it getting back to you. And of course, if you're sending it here, I always ask that you enclose a note with your name and a correct return mailing address, and a detailed list of what you want done. If you send a money order (never send cash) I suggest keeping track of the reciept that comes with it, in the event the package is somehow lost.
Then, close up the box, making sure there's a little "compression" as you close the lids. You want to 'squash' the padding, just a little, since that helps hold things in place, so they don't move around and rub against each other.
Tape the box securely, preferably in two directions. The strapping tape with the fibers is some of the best, and should definitely be used if the package is especially heavy. If not, the wide, clear packaging tape is good. Just make sure you wrap the box all the way around, so the tape holds the top and bottom flaps.

Finally, put a good, legible (readable) address label on the box, including a valid return address. I suggest making sure the label is somewhat waterproof: write it in ballpoint pen, print it out with your computer, or just run a pass or two of the wide clear tape over the label.
I also recommend either a "return reciept" card or the new "Delivery Confirmation" tag. If you go with UPS for some reason, ask for a 'tracking number'.
Then all you have to do is take it to the Post Office, slap a couple of "Priority Mail" stickers on it, they'll weigh it, stick on a shipping label, and send it off.

I have had packages arrive well-packed, but for some reason very wet. Might it have been left out in the rain? A storage room somewhere had a sprinkler accident?
In another case, a gun was sent to me with a baggie full of the trigger components. As I slid the box off the front desk at the post office, I heard a tiny "clink!" I looked down and saw one of the pins for the trigger lying on the floor; there was a hole in the box. I picked up the pin and taped up the box, and very fortunately, all the pieces were still inside. They'd fallen out of the baggie, which had been shredded by the gun itself moving around during shipping.
One package arrived with the gun sitting towards the top of the box, with a layer of wadded newspapers under that, and a towel at the bottom. I E-mailed the owner and asked why he'd put the towel in the box? He told me the gun had been *wrapped in the towel* when he sent it!
AutoCocker beavertails almost *always* get bent. 'Cockers are always arriving with mildly or severely bent 'tails, so keep that in mind as you pack. Make sure the box is long enough.
There have been several times I've received boxes with a hole in the side, from where the feed-neck of the gun had apparently "punched through," probably because the box had been severely dropped or slammed.