That's difficult. Depending on your router's capabilities and what you need from your machine, you can disable SSID broadcasting on the router and also disable file sharing on your Mac. Whilst not perfect, this is a good spot to start. Wireless networks are inherently insecure (perhaps they were not loved by their mothers, who knows) so remember that there's always the possibility that someone can be intercepting your data no matter how well it is encrypted. Having said that, I still do all my internet banking and such on my basic WEP-enabled wireless network. 🙂