I dunno if someone has already suggested this, but I didn't feel like reading a million posts to find out...it seems like this should have come out within the first few posts, and didn't.

If you're designing flash for a client site, and FTPing everything up as you create it- the best approach is to create a 5 second or so "Copyright" message at the very beginning of the flash. Tell the client you'll remove it as soon as you receive final payment. Then once they pay you, just open up the source file, delete the first 60 frames, and publish and upload the file again. Simple.

This way they have no choice but to either pay you or remove your work...no one in their right mind would leave something on their server that blatantly says "Copyright protected by someone else."

Example:
http://www.thatnextbigthing.com/COPYRIGHTleader.swf