As with any sysctl kernel parameters we can change the value of net.ipv4.ip_forward on the fly (without rebooting the system):

sysctl -w net.ipv4.ip_forward=1

or

echo 1 > /proc/sys/net/ipv4/ip_forward

Also RedHat distributions might set this using:

/etc/sysconfig/network:

FORWARD_IPV4=true

and again restart the network service.