The 1-8 kHz range contains only 5% of the power of the voice, but is responsible for 60% of the intelligibility. Some drapes on your side walls, near to the speakers, may help, along with drapes behind your sitting area. These should absorb in the treble region. Try tilting your centre speaker downwards, so that it is pointing at the floor-side wall corner/vertex (?) - this is what the Audioholics/THX guide recommends. Pictures with thick frames can act as good diffusers, helping to add spaciousness to the sound field along with stopping slap echoes. Try to avoid diffusion on the rear wall and on walls near to the sitting area. Diffusion here can ruin the stereo image. Symmetry in treatment is meant to be worth going for.
Lowering any background noise present should help. If you are willing to upgrade your set up, speakers with controlled directivity above 1 kHz are worth going for. This may be to the detriment of a large "sweet spot", but more directive speakers will have less sound energy sent sidewards and upwards. The resultant reduction in side wall and ceiling-to-floor reflections can improve speech clarity.
A small treble roll-off might be worth trying. You can do this using the cinema or THX equaliser on your receiver/processor. Alternatively, I've found a tone control roll-off of 4 dB at 10 kHz works quite well. This is fairly similar to the cinema/THX eq curves.
