Step 1: Find your target
You want to look for "Buisiness Reply Mail" envelopes, which are provided by many companies who want information back from you. These make perfect targets because the amount of money charged to the company depends on 2 main things, Weight and routing code.
Step 2: Mess with the wieght
Step 3: Mess with the routing code
All you need to do is take a black magic marker to this code, and all of a sudden, they loose more money :D.
Step 4: Final steps
Drop into a postal box, and your done.
I do this about once a month, saving up all of the business reply mail envelopes and shredded papers (remember to recycle :D)
I hope you liked my first instructable, Peace.