If using WPA-PSK, use a long key!

If you must use WPA-PSK (meaning WPA with a pre-shared key, rather than WPA using 802.1x authentication via Radius), make sure your key is sufficiently long. Ideally 20 characters or more.

To quote:

Robert Moskowitz’s article, “Weakness in Passphrase Choice in WPA Interface,” describes a theoretical attack on WPA passwords. The tools WPA-psk-bf, CoWPAtty and WEP Crack are implementations of this attack and have demonstrated the ability to break WPA-PSK keys that are 20 characters or fewer. The Aircrack tool suite operates in an active or passive mode to gather the data required to launch these attacks. In passive mode, the Aircrack tools capture the four-packet authentication handshake between an AP and client. The handshake is then processed through a WPA breaking tool for an offline brute-force attack. If the attacker has not captured the handshake, the Aircrack tools active mode will force a disassociation and reassociation.

For more see this article:

http://www.chips.navy.mil/archives/05_jul/web%20pages/Wireless_networks.htm

which gives a fairly comprehensive overview of the challenges here.