Hi,
The obvious thing is to put that line into the /etc/rc.d/rc.local (on redhat, mandrake, and maybe others) or equivalent start-up file.
There is also a setting in /etc/sysconfig/network that does the same thing (IP_FORWARD) .
Remember, the /proc filesystem is just a virtualisation of kernel memory and doesn't exist on disk, nor persist across reboots.
Regards