I find myself forgetting more and more when I use tools like webmin. These usually work with most flavors of linux as well.

sudo nano /etc/networking/interfaces

auto eth0
iface eth0 inet static
address 192.168.3.90
gateway 192.168.3.1
netmask 255.255.255.0
network 192.168.3.0
broadcast 192.168.3.255

Then restart the networking devices
sudo /etc/init.d/networking restart