Memory management code directly from the man at gskinner.com.
import flash.system.System;
import flash.net.navigateToURL;
import flash.net.URLRequest;
...
// check our memory every 1 second:
var checkMemoryIntervalID:uint = setInterval(checkMemoryUsage,1000);
...
var showWarning:Boolean = true;
var warningMemory:uint = 1000*1000*500;
var abortMemory:uint = 1000*1000*625;
...
function checkMemoryUsage() {
if (System.totalMemory > warningMemory && showWarning) {
// show an error to the user warning them that we're running out of memory and might quit
// try to free up memory if possible
showWarning = false; // so we don't show an error every second
} else if (System.totalMemory > abortMemory) {
// save current user data to an LSO for recovery later?
abort();
}
}
function abort() {
// send the user to a page explaining what happpened:
navigateToURL(new URLRequest("memoryError.html"));
}

You can find other system management tips at gskinner.com