If you want to be truly paranoid (like me), you should use WEP keys rotated every so often. The time depends of course on the sensitivity of your data and the amount of traffic you produce. In addition, you should use some sort of VPN solution as well. This provides an extra layer of encryption to protect yourself from sniffing as well as a means of authentication to prevent random people in your parking lot from rying to join in on the fun of wireless.