Enzie:
It is very late here, but in brief:
The principles of atmospheric perspective can be very helpful in creating depth in your paintings.
They
generally dictate that "light objects in recession become darker and grayer" and "dark objects in recession become lighter and grayer".
So, snow on a distant mountain peak is never quite white, and mountains as they recede get lighter and grayer. So, your values closer to the viewer should be stronger and as you recede, lighten your darks and darken your lights and raise the overall values as objects recede and it should appear to have more depth. BTW: It is certainly clear that dark objects go through more value transition than light objects do. So darken your lights just a very little bit, but the effect will be dramatic.
There is no pat formula for this, so you will have to play a little and make your own determinations.
Hope that helps.
