Just to get started with pirate cake decoration, go through various comics and movies about high sea pirates and their adventures and once you have fair idea you can easily incorporate them in you pirate cake decoration.
It will be great if you have friends who are good in making human faces and bodies from edible cake decorative and place them on the top of your pirate cake decorations
Pirate hats, leather jackets, swords, vintage rifles, wrecked ships, golden treasures et al are some other objects that will greatly add flavour of a true pirate cake decoration in your cake.
Other ideas can involve bringing in other accessories in the venue like hanging original swords, or even placing the cake in a treasure box half buried in imitation coins, and displaying other instruments and objects of the pirates like tattered sails, toy skeletons with danger signs, life size dummy pirates with eye patches, high knee boots etc |