Setting resolution
You can set the resolution of the light map on a per face basis to get sharper shadows in isolated areas. As a rule, you should not use values smaller than 4.
To edit resolution an a per face level, go into lightmap mode by pressing shift L. You will now see a checker pattern on the world. This is a representation of the size of the light map pixel density. You can now select the faces you want to edit. You are now able to change the mapping the exact same way that you would edit texture placement.
Optimizing
To make your map as efficient as possible, you should set the light map resolution to lower values for areas that the player cannot go to or see up close.
Bleeding
Light bleeding usually happens at the base of a wall or in a corner. You can avoid this by making sure your wall edges are cut to fit.
Terrain light bleeding is harder to fix. In COD2 this happened mostly with walls that are placed in open areas.
To fix this, the terrain must be made to fit the brush wall.
black or glowing patches
Black or glowing patches can occur on any patch surface, but it mostly happens on decals that are placed on walls.
To fix black or glowing faces, go into light map mode (shift L) and use natural mapping on these patches.
In some cases, there is a decal floating over the surface. To fix this, move the decal so that it is not floating.
Seams and Stretching
Light seams and stretching are breaks in the continuity of the UV’s on a light mapped surface. This is common on patch surfaces, but can also appear on brush surfaces as well.
To fix light seams, go into light map mode (shift L) and edit the mapping until it is continuous.
If you see a static model that looks too bright or too dark, you may be able to fix it by moving it slightly into or out of the direct light.
You can also tweak model lighting using the indirectslightcale and directlightscale settings.