How to improve iptables security for simple rdp forwarding solution?

[p0d]

I created a cron job on my home server to get a whitetlist of ip addresses from my Dropbox account. If there is a new address I add an iptables rule to allow forwarding for that ip to a Windows VM on rdp the port. Other than the forwarding forwarding rules how else could I improve my iptables security?

Thanks,

Chris