You'll need to create ~.Xclients-default if it doesn't exist and change WM to lxsession. Here's my file:
#!/bin/bash # (c) 2001 Red Hat, Inc.
WM="lxsession" WMPATH="/usr/bin /usr/X11R6/bin /usr/local/bin"
for p in $WMPATH ; do [ -x $p/$WM ] && exec $p/$WM done
exit 1