Stuffing Junkmail Postage-Paid Envelopes?

Yesterday I mentioned that I've started returning the postage paid envelopes that come in most junkmail... except I returned them emptya as my personal little statement against the waste in time and resources that they are causing. Many readers emailed to tell me that I only had it half right: I should be weighting the envelopes down and forcing the junk mailer to pay postage on my little care packages. Have others tried this? What works? Most readers had suggestions ranging from sending each junkmailer the contents of a different junk mailers envelope to filling the envelope with shreddings from your crosscut paper shredder. Of course my personal favorite was the guy suggested a few pieces of sheet metal). Take a stand against junk mail! Sorry Mr. Postal Worker!