While no wireless network can be 100% secure, these are steps you can take to make it much more difficult for malicious users to compromise your network.  You’ll need to check the documentation that came with your wireless equipment for details on implementing these changes. 

• Change your access point’s administrator password
  Your Wireless Access Point (WAP) will ship either with a default admin password, or no admin password at all.  Change this immediately!  Be sure to choose a password that you can remember, but would be hard to guess (see Choosing a Good Password from the Computer Science Dept. at the University of Maryland).  Change this password regularly.
  
• Change your WAP’s SSID
    Your WAP will come out of the box with a standard “name”, or SSID.  The SSID will vary by manufacturer; typical names include “linksys” and “default.”  Potential tresspassers know this, and search for WAPs with default SSIDs.  Change this immediately to a unique SSID of your choice.  Change your SSID regularly.
  
• Disable SSID broadcasting
  Most WAPs will broadcast their SSID, inviting any nearby computers to try and connect to them.  Turning off this feature forces malicious users to know your SSID before they can connect.
  
• Enable and require encryption
    By default, all data sent over your wireless network is unencrypted, making it easy for malicious parties to intercept anything and everything you’re sending and recieving over the network: passwords, pictures, etc.  Most wireless networking products will support 128-bit WEP encryption, and many newer products will support the superior WPA encryption.  You should enable the strongest encryption available on your WAP, and require that connecting computers use it.
  
• Enable MAC Address filtering
  This feature allows only certain computers to connect to your WAP, making it much more difficult for tresspassers to use your network.
  
• Update drivers and firmware
  Check with your equipment manufacturer for updated WAP firmware and wireless card drivers. New security features and fixes are often introduced in these updates, so it’s important to keep up to date.
  
• Use secure connections when possible
  For an additional level of security, use secure connections for activities like viewing web pages, checking email, and transferring files.  Many internet service providers now offer SSL-encrypted email and secure FTP services; if yours does not, encourage them to do so.
  
• Run security software and keep your OS patched
    It’s essential that all computer users keep up-to-date with threats to their machines. Windows machines are most frequently targeted for attack, so Windows users must be especially careful to protect their machines against threats and keep up-to-date with patches via Windows Update.  Some good options for free security software for Windows include:
  • AVG Antivirus Free Edition from Grisoft
    A solid antivirus tool, free for single home users
  • ZoneAlarm Basic from ZoneLabs
    A powerful personal firewall tool that helps protect against external attacks and keeps programs on your computer from accessing the internet without your knowledge
  • SpyBot Search and Destroy
    An effective anti-spyware tool

  
  
  Mac OS X users should use the Software Update feature on their machines to download and install security updates.
  

More info:

• Dispelling the Myth of Wireless Security
• Keeping your wireless network secure
• Wireless Network Security for the Home
• Linksys: Important Information for Wireless Products
   
• Securing your wireless network
   
• Exploiting and protecting 802.11b wireless networks
   
• Securing AirPort Extreme Networks with WPA (for Mac OS X)