CRATES!
It costs more, and takes some time to build, but shipping crates are the only real way to make sure. I build mine from framing stud 2x4s and 3/8" plywood. I've never had one of these damaged in transit. Standoffs inside the crates themselves keep things aligned and isolated from any outside impact.
I only ship finished products this way; kits and raw castings are shipped in cardboard boxes.
Overkill? Maybe. I tell my clients that if they want the piece to arrive intact and perfect, the extra $50-60 or so in shipping costs is worth the peace of mind. Sure is a hell of a lot easier than dealing with damages after the fact.
edit: On the topic of insurance: You'll need a sales invoice in order to do anything with this, but even still I've gotten the shaft from USPS because "artwork" is only covered to a finite amount (something like $150 or similar) Also, if you want to get the money from the claim, they'll want the item in exchange for the payout.
I once had a raw casting disappear in transit, and USPS wouldn't pay out insurance because the package wasn't damaged - it was lost. They maintained that wherever it might be, it might still be intact, and therefore insurance wasn't applicable because I couldn't *prove* the item was broken.