I have bought and sold numerous items over the years and have learned the hard way what best can be done to avoid shipping damage.
1. First of all, NEVER... EVER mark a package fragile, or glass, or any other designation that makes it stand out. I was told this by a friend who works for UPS. He said the warehouse people will target such packages, and toss them around just for fun or animosty. Of course, UPS denies this, but I have heard it from other UPS emplyees since.
2. Always exceed the minimum standards for UPS packing. If the item is especially valuable, it always helps to double box the item.
3. The less time they have it in their possession, the less chance it will be damaged. Faster shipping methods are more expensive, but help get it there in one piece.
4. UPS has stated that in the case of firearms, they cannot guarantee that your package won't be stolen by their employees, unless it is shipped next day air. Common sense dictates that if there is anthing about your package, including the shipping address that indicates that it is an item of excessive value, the same would apply.
5. Always provide extra support for any protrusions (i.e: tuning knobs, switches etc.) so that if the box is violently dropped on that side, the protrusion won't take the impact. I have a friend who received a brand new radio with the tuning knob driven clear into the interior of the radio. The foam packing had been reduced to dust by the violent handling.
6. Always use double thickness new boxes when packing heavy items and keep documentation of your purchase. This can save the day, if there is an insurance dispute.
7. Make sure the box you use is rated for the weight of the item.
8. Make sure that you adequately tape the box. I generally tape the long center seam three times with premium tape.
9. Wrap the item with bubble wrap, then allow a minimum of 3 to 6 inches on all sides filled with foam peanuts. Compress the peanuts when closing the box. If double boxing, again allow space all around the inner box and compress foam peanuts before closing.