Lighting is ofcourse a trial and error thing, very much so in Source due to the lack of any real lighting preview.
Recently I've really put allot of thought into how best to light a scene, rather than slapping a few lights in i try to think about interesting shadows and light sources.
[Blocked Image: http://i6.photobucket.com/albums/y226/trioxin/nmrih_mr_feb/nmrih_mr_feb082_chris_citylife_2.jpg]
In this scene I created two windows with an exterior light source to make interesting shadows and contrast in the lighting. Also I used a coloured red light hidden away in a vent shaft (good olde cs style) to give the scene some interesting glow. I then use very gentle point lights of brightness (1 or 2) around and about to give some blue tints.
[Blocked Image: http://i6.photobucket.com/albums/y226/trioxin/nmrih_mr_feb/nmrih_mr_feb082_chris_citylife_1.jpg]
In this scene I tried to have a light source emiiting on all surfaces, but instead of a single big light source I used a combination of spot lights, ambient colour, moon brightness, point light ambients and another classic red styled spot light.
So the TIP here is try to light a scene from more than a single lightsource and colour, then consider how shadows will affect the scene and use high lightmap scales so you get lots of shadow detail.
When it comes to exterior day maps your often limited in terms of light sources, you have the sun and ambient, so make sure the settings you use are as good as you can make them! Instead of simply matching the light direction to the skybox texture adjust the environment light so it gives you the best effect. Slap in an env_sun where it should be and hey presto you have two suns!
I'd be damned if anyone notices, if your that bothered make a new skybox, the lighting and shadows far outweighs the importance of a weird directional sun in the skybox texture.