[CentOS] Rules for port forwarding

Hi,
does anyone remember the rules for port forwarding ?
the followings does not work:
iptables -A FORWARD -i eth0 -o eth1 -p tcp –dport 80 -j ACCEPT
iptables -A PREROUTING -t nat -p tcp -i eth0 –dport 80 -j DNAT –to
192.168.20.1:80

thx lewis