sudo update-alternatives –config x-session-manager
There are 2 choices for the alternative x-session-manager (providing /usr/bin/x-session-manager).

Selection    Path                    Priority   Status
————————————————————
* 0            /usr/bin/gnome-session   50        auto mode
1            /usr/bin/gnome-session   50        manual mode
2            /usr/bin/mate-session    30        manual mode

Press <enter> to keep the current choice[*], or type selection number: 2
update-alternatives: using /usr/bin/mate-session to provide /usr/bin/x-session-manager (x-session-manager) in manual mode

Source: http://crunchbang.org/forums/viewtopic.php?id=37543