I just got back from Shasta...flew up there on United. I think most airlines won't charge if you say it's a trick ski (or any ski. trick ski just makes more sense with the size). I didn't have a problem. I think the weight limit is 50 lbs and it counts towards 1 of your 2 checked bags.
I used the packaging my board came in. If you can get the nose guards too, that's the best. Then I wrapped it in my sleeping bag. Worked perfectly!
One other thing: they said since it was in a box, I had to sign a limited liability form. Not sure if a board bag would have same situation. But, does an airlines really cover much of what they damage anyway???
Check the Southwest web page for sporting equipment.
Hope this helps!