Local news had something the other night about opting out for snail mail offers.
Check out this site
http://www.worldprivacyforum.com

Another thing Iv done, is when I get a preapproved anything from a solicitor, I shred the whole contentsand put it into their selfaddressed envelope and mail it back to them.
Let them deal with the mess.

You can also tape the self addressed stamped envelope to a brick, since 9-11 certain boxes have to be metered so it won't work with a big box with a tire in it anymore. But a brick will still go through as mail and the company that sent the envelope has to pay for the brick.

One of my buddies used to do that until he noticed the bar code printed on some of the return address envelopes.

Some of them actually track the junk mail that they send out and know when you send in the return envelope.