I just play around with the brushes then include some parts of stock photos and play around with the blending mode, colors, opacity,effects, etc. Then i go on experimenting and playing with the images on the sig including the render until i produce a graphics effect which i like.
Sometimes when am lazy to do background gfx, i just smudge on the pic of the render and apply different blending modes.
i don't usually save my sig files so i cant say the step-by-step procedures as to how i arrive at the background. But overall, it would involve not more than 10 different layers.
for the stock photos, i go to these sites:
i use some stock photos to add some texture or details to the sig background.
for the light streaks, apply a white brush over the part of the sig which you want highlighted. Set the blending mode to overlay and play around with the opacity.
hope i have been of help... if you have any more questions, dont hesitate to ask.