A small modification to Ascension's method is to use a 100% white layer(255, 255, 255 rgb) set to Multiply mode. This is especially good if, like me, you have a tendency to use excessively bright and garish colors.
Astrographer - My blog.
-How to Fit a Map to a Globe
-Regina, Jewel of the Spinward Main(uvmapping to apply icosahedral projection worldmaps to 3d globes)
-Building a Ridge Heightmap in PS
-Faking Morphological Dilate and Contract with PS
-Editing Noise Into Terrain the Burpwallow Way
-Wilbur is Waldronate's. I'm just a fan.