http://www.netbsd.org/guide/en/chap-x.html#chap-x-mouse

Gives this example:

Section "Pointer"
  Protocol    "wsmouse"
  Device      "/dev/wsmouse0"
  ZAxisMapping "4 5"
EndSection    

With XFree86 4.x, I believe it should be:  

Section "InputDevice"
	Identifier  "Mouse0"
	Driver      "mouse"
	Option	    "Protocol" "wsmouse"
	Option	    "Device" "/dev/wsmouse"
	Option	    "ZAxisMapping" "4 5"
EndSection

At least, most of that was generated by xf86cfg.  

--jkl