Those are good tips from tryo. There is also a longer tri-corner box from USPS that is 36 inches long. I frequently use that. You may have to ask them to go in the back to get them, because sometimes they don't have them out front at the post office. Most 1 gallon trees will easily fit 2 or 3 in such a box.
The great thing about the USPS boxes is that they are free!
If you have a larger tree you can actually tape together two or more of those tri corner boxes in various ways. Not sure if you're technically supposed to do this, and often times the lady at the counter gives me dirty looks, but in the end they ship them just the same and are still free.
I have shipped much larger trees across the country in various shapes, sizes and varieties of gnarly taped together boxes. Most times it gets there fine, even if it's not the most attractive solution.