Ip forwarding

Hi,
I know there's much out there about this topic but I'd like you to
clarify this to me.
I've got 2 network interfaces:
a) 192.168.0.1 mask 255.255.255.0
b) 192.168.1.1 mask 255.255.255.0

and I would like packets incoming on interface (a) to be forwarded to
(b) and vice verse.

I tried with
echo 1 > /proc/sys/net/ipv4/ip_forward
echo 1 > /proc/sys/net/ipv4/conf/wlan0/forwarding
echo 1 > /proc/sys/net/ipv4/conf/eth1/forwarding

but still nothing good.. I guess I have to use the iptables tools but
I don't know much about their usage...
Could you lend me a hand?

Thank you guys
Riccardo
.