// params: eventName, listener, capturePhase, priority, useWeakReference
someObj.addEventListener("eventName",myFunct,false,0,true);

http://gskinner.com/blog/archives/2006/07/as3_weakly_refe.html

GSkinner show's how to set up a weak listener that way Garbage Collection can come around and recollect some memory when it needs to. 

Here's another resource that gives a more in depth look:
http://createandgrow.com/blog/?p=6

AS3: Weakly Referenced Listeners

ActionScript 3