I like a static address for servers, by default my install of Ubuntu has a dynamically assigned address. To fix this I simply used my favourite text editor to change the settings in /etc/network/interfaces
Replace the line
iface eth0 inet dhcp
with:
iface eth0 inet static
address required ip address
nnetmask required subnet mask
network required network address
broadcast required broadcast address
gateway required gateway address (i.e. your routers address on your LAN)
Replace the text in bold with your particular network details.
For your new settings to take effect issue the following command:
sudo /etc/init.d/networking restart