WEP is a good idea but one of the most sure ways to make sure your wireless network isn't penetrated is to control signal leakage. Can't hack the network if you can't pick it up. To that end, tightly controlling power levels (on APs that allow it) and using highly directional antennas can mitigate much of the threat. For that last bit, there's a new way to make your network 'invisible' that I plan to try out just as soon as I can throw up another Linux box at the office called FakeAP. Basically it emulates THOUSANDS of access points. The chances of anyone actually finding your network is slim to none.