1. First open a new document that is the size you would like your sig to be, I used 200x400px, and make sure your colors are black and white by pressing (D). And go to Filter > Render > Clouds. Press ctrl+f to randomize this until you get a design you like.
2. Take an abstract brush set and go crazy on one side of the sig with a white fill color, so that it looks like it is fading towards the black clouds. Right click the layer and click blending options. Enable the drop shadow at default settings to give an added effect.
3. At this point you should have a render or stock photo in mind. I am going to use a sonic render from planetrenders.net. From this render I am going to make the background into a blue color using a Color Balance Layer (Layer > New Adjustment Layer > Color Balance...). Play around with the colors until you are satisfied.
More Tutorials: 
