I run this script in my .xinitrc to reduce the cursor speed for my 3600 dpi mouse:
for i in `xinput list|grep "4Tech USB Full Speed"|cut -d "=" -f 2|cut -d " " -f 1`; do
xinput set-float-prop $i 'Device Accel Constant Deceleration' 2.5;
Replace "4Tech USB Full Speed" with your device name (run xinput list).
Doing it like this has the advantage that other mice that get connected (or touchpads) aren't affected.