Particle System Advice?

[thetastyfish]

Hello everybody! I am about to start adding particle systems to my levels to give them that much needed creepy atmosphere. Before I get too far along though, I'd love some pointers. What should I avoid, and what should I strive for? Like, how would I make a basement look like a basement without just filling it with clouds of dust, and at the same time avoiding being too scarce? How will I know when I've reached that perfect amount of particle system? Thanks so much!