As stated by jpkotta, network-manager is likely the culprit.
You can see its status by running
ps -aux | grep network-manager | grep <username>. If you get a result, it is running, otherwise it isn't.
It will keep overwriting any changes you make with ifconfig as long as it is running.
Kill network-manager by running
sudo service network-manager stop.
You can bring it back up any time with
sudo service network-manager start.
Once it is disabled, use ifconfig to set your static, OR edit your
/etc/network/interfaces file to include something like:
iface eth0 inet static
ifup -a to bring up the interfaces you have in your
All of this can be avoided though, if you'd rather not mess around with killing network manager. Just click on its icon in the taskbar and click 'edit connections'.