If you usually start your X server by writing $ startx (thus you don't use a graphical login like GDM or KDM) you need to edit the .xinitrc file placed in the home directory. Create immediately it if you don't have :)
$ touch ~/.xinitrc
$ emacs ~/.xinitrc
write the name of the DE or WM you wish to start, here it's an example of the content
#PROGRAMS TO AUTOMATICALLY START
#DE o WM da avviare
if you want to switch from kde to gnome just comment startkde and ucomment gnome-session.