Before setting your interface into monitor mode and trying to use any of the aircrack's wireless monitor and injection tools (ie: airodump-ng, aireplay-ng), always stop all network related services.
This is the command I use to stop all networking services and starting airmon:
sudo service avahi-daemon stop ; sudo service network-manager stop ; sudo wpa_cli -i wlan0 terminate ; sudo airmon-ng start wlan0
This is the command I use to stop airmon and starting all networking services back on:
sudo airmon-ng stop wlan0mon; sudo service network-manager start; sudo service avahi-daemon start
(Note that "wlan0mon" is the new notation for interfaces in monitor mode, starting with version 1.2-RC1. For previous versions, use "mon0")