Try making the skybox a dark one, then setting the environment lighting to some sort of mid to dark blue, with the ambient a slightly lighter colour maybe even a purple. Play around with the shadow_control (or whatever it's called) entity too.
Make all your lights come from specific sources: you might have street lights, car lights (used a lot in the official maps), Shop window displays, apartment lights, floodlights. Vary it up a bit too, maybe light from a fire, light from a siren or alarm, neon signs or just signs with lights on them, traffic lights...
Also don't use the normal light entities for everything, use light_spot instead, I used to place one aprox two metres off the ground facing down and another aprox 1/2 metre off the ground facing up, made some good effects.
Then again if you do use lights use the fade distance (I think it's called) the thing that has a 0% and 50% value in the light properties. Try those with different numbers and you'll work out soon enough how it works. (Sorry if any of this is wrong haven't opened hammer in a year
) Good luck!