Be sure to pack it very well... remove fins etc.. (if you can't then the polystryine backing you get round TV's and things are creat to cut into a protective cove.
Wrap the board bubble wrap or towels/clothes etc
Its also a good Idea to put on the outside of the board bag "Top Load Only Please" in your language and destination language.
Most airlines are fairly sound about taking boards on flights. And board tax wise its a yes and no to declaring it before you fly. sometimes the'll tax you for it if they know its comming but then some will just run it through regarless.
In the UK Ryan Air and alike cheap airlines do have a standerd charge on boards each way esp to destinations well known on the surfer travels eg berritz, portugal, spanish west coast.
Your best but is to check it in before hand really atleast then you know for sure.!