I have configured my Logitech Marble Mouse trackball as follows (I am using
the trackball with my right hand).

Button          Action
----------------------------------------------------------
Left big        X11 mouse button 1
Right big       X11 mouse button 3
Left+right big  X11 mouse button 2
Left small      Scroll wheel emulation with the track ball
Right small     X11 mouse button 6 (back button on Firefox)

I have the following block in my /etc/X11/XF86Config:

Section "InputDevice"
        Driver "mouse"
        Identifier "Trackball"
        Option "Buttons" "4"
        Option "Device" "/dev/psaux"
        Option "Name" "AutoDetected"
        Option "Protocol" "MouseManPlusPS/2"
        Option "YAxisMapping" "6 7"
        Option "EmulateWheel" "1"
        Option "EmulateWheelButton" "4"
        Option "EmulateWheelInertia" "12"
        Option "Emulate3Buttons" "yes"
EndSection

and in addition one needs to remap some of the keys with xmodmap, e.g.
by:

xmodmap -e 'pointer = 1 2 3 7 6 4 5'
-- 
Ville