If you use ifconfig to alter the MAC address, the change is not preserved (reboot). If you want the MAC to be changed permanently, you need to edit the interface's config file.
In Debian/Ubuntu this is /etc/network/interfaces. Add a line like hwaddress ether 01:02:03:04:05:06 for the interface in question:
iface eth0 inet dhcp
hwaddress ether 01:02:03:04:05:06
Best, PEra