Flash crowds and high concentrations of users in wireless LANs (WLANs) cause significant interference problems and unsustainable load at access points. This leads to poor connectivity for users, severe performance degradation, and possible WLAN collapse. To validate this claim, we present two case studies of large, heavily loaded operational WLANs. These studies provide significant insight into the degraded performance and collapse of a WLAN during heavy use. To address these problems, we propose IQU, a practical queue-based user association management system for heavily loaded WLANs. IQU grants users fair opportunities to access the WLAN while maintaining high overall throughput, even when the WLAN is heavily loaded. The basic premise of IQU is to control user associations with the WLAN through request queues and work period allocations. We implement a prototype of IQU and evaluate it on a wireless testbed. Our evaluation demonstrates that IQU significantly improves network throughput under heavy load; the tradeoff is that users have to wait for network access. We explore the impact of IQU parameters on system performance, and validate the robustness of IQU under heavy load conditions. Through IQU, WLANs can be utilized efficiently and network collapse prevented.