use less blocks. fill in detail block work with 'detail' flagged blocks, map entities and models. Don't use 'detail' flagged blocks except when they are layed on top of normal block surfaces.

use that snap to grid and snap yoru blocks. but DON'T overlap/intersect them

don't leave gaps in your level blockwork that opens to the infinate engine level

texture all surfaces that will NEVER been seen with a texture that has the NONE flag set

DON'T use CGS subtract, always build blocks, never cut

avoid multiple angled surfaces in a single block. as many of each block surfaces should be aligned ( and snapped) to the level grid) as possible

all these will help reduce your build