4th thing module for iptables must surely be loaded (modprobe)
5th thing goto
www.linuxguruz.org/iptables -for firewall examples
6th thing in iptables you need minimum of three entries for sharing
..check this example
echo "1" > /proc/sys/net/ipv4/ip_forward
iptables -P FORWARD DROP
iptables -A FORWARD -s $localnet/24 -j ACCEPT
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -t nat -A POSTROUTING -s $localnet/24 -j SNAT --to $ipofIneteth
(localnet=192.168.1.0 -for example)
ipofIneteth=123.123.123.123 -your external ip
if you have dhcp use last line like this
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE (if eth0 is your outgoing device)