The most important rule of shipping things, especially large, heavy, expensive things, is:

Expect your package to be dropped from a height of at least five feet at some point in its journey.

I once saw a delivery guy drop a package containing a $30k camera from his chest height.

So, with that in mind, protect your package.

1. Non-framed prints should be in a mailing tube. Put two mailing tubes if the print is valuable or you're paranoid.
2. Framed prints should be wrapped in bubble wrap at least three times around (sharp corners = broken bubble wrap, if you don't have cardboard covers for the frame)
3. Fill extra space with crinkled up paper.
4. Make sure that there is at least an inch or two (preferably more) of space around the print itself, and that that space is filled with packing material.
5. Do not let things shake around inside the box. Contents have a tendency to shift during flight, but things that are locked down won't move as much, and therefore have less chance to break.
6. If you're not confident in your packaging, insure it.
7. If you absolutely must have your package arrive safely, insure it.

For that size print, I'd do these steps:

1. Wrap the (framed) print in a single piece of butcher's paper.
2. Wrap that package in several layers (four times around ought to do it) of bubble wrap.
3. Crinkle up some butcher's paper. Put that into your cardboard box.
4. Place print into cardboard box. If there is space on the sides, fill those spaces with more crinkled up butcher's paper.
5. Top off with butcher's paper.
6. Use packing tape to close the ends of the box. Make sure that you use enough; a single piece of tape around the edge is not likely to do it.

Maybe it's overkill, but doing this has made most of my stuff arrive intact.