On a Debian box, I have installed IceWM and IceWM-lite, I'd like it to boot with IceWM-lite by default.
So far I have done this through the desktop menu after each boot.
If you have GNOME installed, you probably use gdm as display manager. Actually it should have remembered your default session, but you can do it manually by setting the file ~/.dmrc:
[Desktop]
Session=icewm
Language=en_US.UTF-8
If you don't use display managers, add the following line to ~/.xinitrc:
exec icewm-session