This is what worked for me. 

I'm running a Redhat linux, kernel 2.4.20-8, iptables v1.2.7a

Hopefully it will help you out too.

$IPTABLES -t nat -I PREROUTING -p tcp --dport 6881:6999 -j DNAT --to-destination [IP of machine running BT on local network]

$IPTABLES -t nat -I PREROUTING -p tcp --dport 6969 -j DNAT --to-destination [IP of machine running BT on local network]

$IPTABLES -A FORWARD -p tcp -i [external infterface] --dport 6881:6999 -d [IP of machine running BT on local network] -j ACCEPT

$IPTABLES -A FORWARD -p tcp -i [external infterface] --dport 6969 -d [IP of machine running BT on local network] -j ACCEPT

http://wiki.etree.org/index.php?page=BitTorrent_iptables