I guess I could put a random filter trick here on this thread.
Particle depth tricks: fake cast shadows
Since actual cast shadows are hopelessly slow, you can use fake shadows to add depth to Bomber particles when the particles "float in space."
Actually, a couple of different things have been used here to add depth:
1) Depth sorting with a perlin noise input
2) Scaling with same perlin run through a tone curve for better distribution (if you use the perlin as is, you get too many large particles)
3) Making "shadow" particles and compositing them between other particles.