This is somewhat easy, but can be somewhat time consuming when you have many textures to change...
When applying textures to your brushes, set all faces that can't be seen by players to the null texture. Note: Don't apply null textures to unseen sides of sky brushes since that might create a problem with environment lights not working, also if you by accident apply a NULL texture to a face that can be seen then then it will, depending on how your gfx card and drivers work, display as black or with a tiling effect like what you would see if you were nocliping.
Doing this will reduce the bsp's size when compiled and reduce w_poly in r_speeds a little bit possibly making your map a little faster, in terms of fps. There are some example pics below.
The null texture forces the compile tools to discard faces when compiling the map.
Polygon view with NULL textures. There would have been more polys shown if NULL textures weren't used.
Faces viewable to players.
Faces not viewable to players.
