The method i use is derived from an old method by ryan clark.
http://www.zarria.net/nrmphoto/nrmphoto.html
He uses a flashlight to photograph an object, then combines the “lightmaps” together to make a normal.
I also combine it with the rgb lighting method that is common in the movie industry to combine multiple light directions into 1 render. Its basically just a normal rgb light rig in your 3d program. Left light is red, top is green, front is blue. (dont need front for the normal) You need to do a second render pass for right and below, (right=red, below=green) You can also add a blue back light into your second render but its not needed for the normal. (front and back lighting useful for the diffuse though)
When setting up the lights in your scene, make them exactly 90 degrees on their respective axis as 90 degrees seems to yield the best/most accurate results. There are times however what all the side lights will not light a section of the sim. (ie: a cavity in the middle of the cloud) These areas will show up as essentially flat on the normal. There ways to minimize this, either by thinning out the sim so the light penetrates more, or by rendering a second normal map with lights that will cover those areas (45-60 degrees instead of 90) and then combining the two photoshop.