How should I pack my miniatures?
The key to safe shipping is minimizing space, and immobilizing. A larger box, paradoxically, is less safe for your minis than a smaller box. Because of this I strongly recommend removing any minis from their original packaging - these just take up space, forcing you to use a larger box.
When packing your minis, the key to safety is immobilizing. The outside world is far less dangerous to your minis than they are to each other. As long as you can pack them so that they cannot slosh around within the box, even fragile gnoblars can ride safely with landraiders.
This is especially the case if you are shipping metal minis. Immobilizing prevents them from behaving like shrapnel bouncing around in a tank. Do not use styrofoam peanuts for this, they behave like a fluid and let the metal bounce around anyway. Just use scrunched up newspaper.
Plastic Sprues
Remove the original box, and flatten it. The picture guide on the box is usually helpful, so including it at the bottom of your box is a good idea. Rubber band the sprues together, and they'll be fine.
Assembled Plastic Minis
If you are only shipping plastics and hardly any metal or other heavy minis at all, if you just put these into a plastic sandwich / ziploc bag they should be fine. If you are shipping a fair number of metal minis or any metal tanks or such, you must make sure you pad between the minis, and pack them so that they are immobilized.
The padding helps protect the plastic minis from the weight of the metal ones, this is really quite minor compared to the risk of metal bouncing around like shrapnel. Immobilizing is the key.
Unassembled Metal Minis
These are pretty indestructible. Just remove the parts or figs from their blisters and boxes and just stick them in plastic bags, the original box flattened and placed at the bottom of your package helps as a picture guide for your painter.
You do want to immobilize these if you can help it, just use newspaper.
Assembled Metal Minis
As with assembled plastic, exercise caution. If its a mini that's asking to break, you want to wrap it up, especially Infinity figures. If its good, sturdy, plastic, you may be able to just stick it in a plastic ziploc along with thirty others and they'll all make the trip fine... As always, try to immobilize.