You need to put a script in /etc/network/if-up.d/
Start a text editor as root (kdesu kedit will do it) and type the following:
# My simple script to change the mtu to 576
ifconfig eth0 mtu 576
Save this file in /etc/network/if-up.d/ with any name you like - I'll use myscript for this example.
Change the permissions to allow execution e.g.
chmod 755 /etc/network/if-up.d/myscript