I found the default mouse acceleration to be way too fast (particularly when trying to click on the single-pixel window borders in xfce).
To slow it down, I followed Patrick Mylund's instructions. These are the results specific to the Logitech G400.
$ xinput --list --short
This shows the names/IDs of input devices. In my case, "Logitech Gaming Mouse G400".
Now create a file ~/.xinput-mouse.sh, chmod it +x to make it executable, and add edit to include the following command:
xinput --set-prop "Logitech Gaming Mouse G400" "Device Accel Constant Deceleration" 4
Add a file xinput-mouse.desktop to ~/.config/autostart with the following contents:
[Desktop Entry] Encoding=UTF-8 Version=0.9.4 Type=Application Name=xinput-mouse Comment=Slow the mouse acceleration Exec=/home/<username>/.xinput-mouse.sh OnlyShowIn=XFCE; StartupNotify=false Terminal=false Hidden=false