WLAN Security

I currently I am using four Cisco 340 aironet wireless access points and one Linksys wireless access point for our wireless infrastructure. What is the best form of security to have besides using WEP. I don’t feel confortable using just WEP, sinceI have found some network sniffers and can decrypt the WEP keys. We plan to expand our WLAN and I want to make sure we have it secure enough so “Joe Blow” can’t sit in his car in the parking lot and get access to our network.

Thanks,