If you use a Fritz!box modem, you can configure the DynDNS section to update your account:

1. Access the Internet>Permit Access menu, click the DynDNS tab.
2. Select User-Defined as the DynDNS Provider, then enter the username, password and hostname (e.g MYDVR) of your account.
3. Enter the Update URL as:<username>&p=<pass>&ip=<ipaddr>

Click 'Apply', and thats it, now you can forget about manually having to update your ip address.