What I used to do before getting a little better, was to generate clouds, and them with the threshold i started molding the continents (all of this in PS, and also saw this technique in Tear's tutorial!).
But one day brownsing CG Textures, I saw this textures of painted metal, and never really worked on something, but I think it has a lot of potential, with the landmass and oceans
[CG Textures] - Textures for 3D, graphic design and Photoshop! go to Textures - Metal - Painted

Example: MetalPainted0101