The easiest solution, IMHO, is to configure the WHS for a static LAN IP address. See this article for help setting that up.
http://support.microsoft.com/kb/323444
Just make sure it is outside the range of addresses your router DHCP server assigns. With DD-WRT I believe that range starts at 192.168.1.100 and goes up. For example you might select 192.168.1.10 for your WHS box.
DD-WRT, at least the last time I ran it, also has a function to reserve a DHCP assigned IP address based on the clients MAC address. I believe DD-WRT called that static DHCP. Check the DD-WRT documentation for further help.
Lastly you can probably change the DHCP address lease time for the DD-WRT DHCP server to a longer period. I don't know what the default is so check the DD-WRT documentation, etc.