Ya know, I'm just starting out on it too, but I find (once you get the shape ya like and get the white down of your island/continent) I like to shrink the outline by a few pixels(depends on the size of the image), Smooth it by half, and then Refine Edge(at least that's what it's called in Photoshop) with a contrast set to 100%. Seems to clean up the lines a bit. In fact if you ever run into issues with an outer glow or a stroke because it's bleeding in funny ways, ctrl+click the layer icon and refine the edge with contrast at 100% - seems to do wonders cleaning up messy blurring/feathered areas.