DynDNS provide a really useful service as DirtyDamnApe suggests above, however, the service is predicated on some machine inside your home network being able to determine the external IP when it changes. If your router/firewall is a programable machine this is usually not a problem as they have a selection of user written (quality and facilities vary) update agents available for download from their website.

What can be a problem is finding a way to determine the external IP from stand-alone firewall boxes like SonicWall etc.

If your lucky, the box will have a html status screen--available from its internalip:port 80 (or 8080)--that will list the current external IP. Writing your own agent to extract this and update the DynDNS profile is reasonably simple to do.

You may find, (as I did) that you need to download the latest firmware for your firewall box in order to get the latest version of the status screen that has the external IP displayed.

Good luck.