I use no-ip.com, although I know there are several alternatives out there. You can simply apply for a free account online, then you do
apt-get install no-ip
# to configure
no-ip -C
# enter your email, password for no-ip
/etc/init.d/no-ip start
In your /etc/inittab, you will see a line like
id:5:initdefault:
which tells you what your default run level. In this case, my run level is 5, so make sure you see S20no-ip in your /etc/rc5.d
