Open wireless networks can give the right individual an opportunity to access the Internet in a way that makes them virtually untraceable; most importantly you are exposing pretty much anything you do on your computer that involves the network! Here are a few simple tips to minimize potential threats to your wireless network:

• Enable WEP (Wired Equivalent Privacy). Even though WEP is quite weak and hackable, it still provides a first level of defense by encrypting the traffic between you and your access point. Using 64-bit WEP you can gain some security benefit without slowing down your network noticibly. You could also use WPA, a cousin to WEP that's tougher to crack. Make sure both your router and wireless driver support it before implementing.
• Remove your SSID (service set identifier) altogether. Especially if you only have a couple of computers; set the network name manually. Makes the network difficult to use if it can't be seen.
• ALWAYS change the default password on your router! The default password of most all network equipment can be found with a simple query to Google.
• Enable MAC address filtering. Using this, only your unique wireless cards can communicate with your router. This does add the burden of manually needing to add your buddy's laptop to the network so you can get your game on.
• Turn off your router when you don't plan on using them. Why risk being hacked into if you're not using your wireless network?

Just a few tips from your uncle Larry :-P