Install package ifenslave-2.6.
aptitude install ifenslave

make sure the real NICs kernel modules are loaded automatically
modprobe --list | grep -i bonding

edit /etc/network/interfaces and replace all references to eth0 to bond0. Also, replace “allow-hotplug” with “auto”, and save the file. You can use sed to do all this in one command:
sed 's/eth0/bond0/;s/allow-hotplug/auto/' </etc/network/interfaces

Execute the following commands:
echo "up /sbin/ifenslave bond0 eth0 eth1" >> /etc/network/interfaces
echo "down /sbin/ifenslave -d bond0 eth0 eth1" >> /etc/network/interfaces
echo "bonding" >> /etc/modules
echo "bonding mode=1 miimon=250 primary=eth0" >> /etc/modules
The above commands are equivalent to opening each file separately and adding the quoted lines manually.

Stop eth0:
ifdown eth0

And restart networking:
/etc/init.d/networking/restart