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