The problem with WEP is that the implementation of its cipher is broken. Without getting too technical, if enough traffic is going over the network, part of the key gets duplicated every so often. With enough duplicate keys, you can reconstruct the entire key very quickly–usually a matter of minutes with publicly available programs like Kismet, KisMAC, and aircrack-ng.
The way to protect against this is to not use WEP. Instead, use at least WPA, which is much harder to crack, and choose a secure password.