iptables help

I want to redirect all outgoing HTTP traffic to a Squid proxy. I tried:

$IPTABLES -t nat -A PREROUTING -s 192.168.2.192 -p tcp --dport 80 -j DNAT --to 192.168.2.231:3128

to test my intended rule:

$IPTABLES -t nat -A PREROUTING -i eth1 -p tcp --dport 80 -j DNAT --to

However, it doesn't work... I can no longer access web pages at all, and Squid isn't seeing my requests at all. iptables -L doesn't show anything that has to do with the new rule. So, I botched something, but aren't sure what :-)