Using the clouds filter is very versatile. Many people use clouds (and difference clouds) to make landmasses coupled with a threshold; this technique allows easy creation of randomised (and thus more realistic) coastlines. The threshold will you to create a black and white image (where often black is sea and white land). You can then use masks, and all sorts of techniques, to add colours, textures and so forth to represent terrain.
I highly suggest that you follow Ascension's Atlas Style Tutorial for Photoshop. Even if its not a style that you like the techniques you will learn will be quite helpful.
Tear also has written an excellent tutorial - Saderan.
If you need some more help, don't hesitate to ask (PM me if you want)!