To save myself some typing, I usually do the following:
prompt> sudo passwd root
prompt> Enter new UNIX password: ***
prompt> Retype new UNIX password: ***
prompt> su
Password: ***
prompt>
Granted, I wouldn't recommend a 3 character password if you have any services enabled which are remotely accessible by root...but if I have to boot off a LiveCD its most likely because my system is b0rked anyhow.
$0.02