I use dyndns.org and here are the parts that aren't commented out (this is with an interface-based setup rather than web check though):
Code:
daemon=300
syslog=yes
pid=/var/run/ddclient.pid
use=if, if=eth0
login=username
password=password
server=members.dyndns.org, \
protocol=dyndns2 \
hostname.mine.nu
Read the documentation and comments in the example ddclient.conf, it's not that hard to figure out.
Håkan