What has been mentioned is pretty much all there is to it. I like to package each beer in a 1 gallon zip lock baggie. So if a bottle breaks, the beer is contained. I have had a package with a broken bottle get all the way to me from another shipper, I have had a package with a broken bottle get to another brewer and not know until the box was opened. On the other hand I have had a package get held up because it was leaking...I had to call and get that all straightened out. Just needed to approve the "deliver it anyway".
They do have a don't ask, don't tell policy. So with that said, do not do what my wife did when she offered to ship off my beers to a competition via FedEx. When asked what was the contents, she did say beer. Not only was she embarrassed at the counter, but quite ticked off at me. I figured she would know this by now. When I have been asked what is packaged, I often say "books", or "glassware" and even "canned goods". Be careful with saying glassware because unless they package it up, they can not guarantee the shipment so they may ask to inspect it. Or con you into placing your package in another box with packing that they CAN guarantee and are willing to ship. It cost me a little extra money, but I wasn't about to have him open and inspect my contents.
It's not that big of a deal, keep it simple and smart if asked what is the contents. Only use UPS or FedEx.