This paper contributes to the definition of an association policy for a multi-channel, multiple AP WLAN that depends on both physical rate and realised throughput. We show that existing proposals are inefficient when several APs share the same channel and define a new policy that is demonstrated to be close to optimal. Policies are compared through their traffic capacity defined as the network stability limit. We determine this capacity analytically using the fluid limit method for some simple network configurations deriving insight into the structure of the optimal policy. The quasi-optimality of our proposal is verified by simulation of a more complex network configuration.