I just got a test machine at work running Debian. It was set up to use dhcp and for development purposes I needed a static IP address, so in order to change that I needed to do the following:

1. kill the dhcpclient to preserve the DNS settings kept in resolv.conf
2. type: ifconfig eth0 <your static IP address> netmask 255.255.0.0
3. followed by: route add default gw <gateway ip address>