sudo apt-get install ddclient
sudo nano /etc/ddclient.conf

## OpenDNS.com account-configuration
##
use=web, web=myip.dnsomatic.com
ssl=yes
server=updates.opendns.com
protocol=dyndns2
login=opendns_username
password=opendns_password
opendns_network_label

Now, just to be sure everything sticks I generally reboot at this point, then check the status afterward:

sudo /etc/init.d/ddclient status

You should see:

Status of Dynamic DNS service update utility: ddclient is running

However, that may not always catch problems in the ddclient.conf file, so issue:

sudo /etc/init.d/ddclient restart