This is the best example I could fiind of the technique I'm talking about. By mixing the properties of the layers and differing the opacities by over laying the layers (much like how you use watercolours) you get this kind of effect and you can alter the depth by altering the opacity's without having to use the history brush...
http://www.eatsleepthink.co.uk/iomega/images/misc-7.jpg">
