Build a big fire on the highest shore point; if you know where the shipping lanes pass, put your fire in the most visible spot. When you see a ship or a plane, you can create big amounts of smoke by tossing on slightly damp moss or branches. Create several fire piles that you can light quickly. To attract attention from the air, arrange rocks that contrast with the sand into a very large pattern that would not occur in nature. You can also root around on the island for materials that will reflect the sun and use them to signal passing planes or boats.