The way that I do it is to use the clouds filter, then lighting effects, delete the darks, then use the lasso tool (with a feather of 50) and just grab a chunk of mountain and drag it to where I want it. It's not a true DEM but it lets me put mountains where I want them despite starting out totally random. Hand-painting a DEM in PS is a very long and tedious process - I've never had the patience to actually finish one.