Setup Static IP Address on Debian 6 (Squeeze)

During installation, Debian uses DHCP by default.You should disable DHCP feature for primary network interface by editing /etc/network/interfaces
By default, it looks like following:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
auto lo
iface lo
inet loopback


# The primary network interface
allow-hotplug eth0
iface eth0 inet dhcp

We should make this look like:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
auto lo
iface lo
inet loopback


# The primary network interface
#allow-hotplug eth0
#iface eth0 inet dhcp
auto eth0
iface eth0 inet static
address 192.168.1.100
netmask 255.255.255.1
network 192.168.1.0
broadcast 192.168.1.255
gateway 192.168.1.1

You may not need to enter network and broadcast details.
Now, restart service by 

/etc/init.d/networking restart