If you set up a wireless network, it is very important that you use all of the security capabilities of the devices. These include:
Use the strong (128 bit) WEP encryption built into the devices. Some products only support 64 bit encryption, but most wireless devices support both 64 and 128 bit encryption. Use WEP for authentication AND to secure the data being transmitted. This can reduce network throughput by 10-15% (for 64 bit) or 15-25% (for 128 bit). This is not bullet-proof protection, but it requires significant effort to break. Someone would have to eavesdrop on a LOT of your network traffic in order to effectively crack this encryption.
Secure your Router/Access Point. Make sure that you require a password for configuration of the router or access point. In addition, make sure you change the default password, as these are well-known. You should physically secure your Access Point if possible, to prevent someone from performing a "hard reset" and setting the passwords back to their defaults.
If possible, disallow remote configuration. This requires you to plug directly into the Access Point/Router in order to configure it.
Use MAC address-based association and access control. The MAC address is a unique identifier in a device, and by limiting access to certain MAC addresses, you can help lock out intruding computers. This can be done on our wireless router and our broadband router.
Use NetBEUI instead of TCP/IP for file and printer sharing. If someone gets access to your network, they won't be able to access your files, just your internet connection.
Only share what you need to. (share folders and files, not entire hard drives).
Secure your sensitive files with a strong password. (Not a dictionary word, containing non-alpha numeric characters like: !@#$%^&*).