DNS-O-Matic is a free service provided by OpenDNS. With DNS-O-Matic, you can propagate IP address changes across multiple services such as DynDNS, OpenDNS and many others.
With DD-WRT, you can set-up DNS-O-Matic so that your services stay updated with your IP changes as soon as they happen.
This article will very quickly go through the process of setting DNS-O-Matic on your DD-WRT powered router.