When you are trying to track down a bug in a SWF, it’s a common practice to add trace statements to the FLA. By tracking various values in the program, you can usually find the bug. Unfortunately, it only works reliably when you publish the SWF from the IDE. There are cases where this isn’t feasible. For example, if you need to see whether or not flashvars are being received correctly by your SWF.
Debugging with Vizzy is more reliable and faster than Flash’s own remote debugging (which often crashes the browser). It’s already become an indispensable part of my debugging toolkit. In case you missed the link above, you can download it here.
If you are having trouble getting it to work correctly, check out this page of the wiki. I have also noticed that Vizzy locks up sometimes when I have Photoshop open. So, you may have to close photoshop to do your debugging.