I don't think you've said if you're using multiprocessing or not.  If you've got the RAM, try setting it to 3 GB per CPU and increasing the CPUs reserved for other applications so it shows "Actual CPUs to be used" to 2 or 3.  You also want to reserve enough RAM for other applications so you don't run out of physical RAM space.  If you're not doing this already, it throws another CPU at your render which can significantly increase speed, provided you've got the RAM to cover it.

A lot of our artists here seem to be very anti-multiprocessing, but that seems to be due to bad experiences with it in the past or it being mis-configured.  They'll claim it makes renders slower.  I'm a big proponent of it in most instances and have a whole thing I've typed up about how I like to configure it and why.

Alternately, if you've already tried multi-processing and you've got extra machines sitting around with all the plugins, fonts, etc., and your project is network-accessible by all machines, you could do a multi-machine render to an image sequence by checking "Skip Existing Files" in the render settings.  Multi-machine seems to work better with multi-processing turned OFF for some reason, so you're in the realm of launching multiple command line renders on each machine to use more than one thread to render with.

If you haven't tried multiprocessing -- really, try it. :)

 - Nathan