Mappers, visit all of these links - they're important!
LEAKS
OPTIMISATION (func_detail and more):
Youtube.com
Youtube.com
Skyboxes
Lighting
Constant-Linear-Quadratic Falloff
The Source Sky Texture List (includes best settings for light_environment)
More in-depth light_environment tutorial(s) by 3kliksphilip here.
Like the map in my desktop wallpaper? You can download it here.