Over the last two months, I've been doing an experiment and I'm happy to say it actually seems to be working. I have been clicking on the "unsubscribe me" link in every piece of unsolicited (or possibly solicited but no longer wanted) email I receive. As of the last week, I've noticed a marked decrease in the amount of such mail.
I started doing this today on every piece of unwanted marketing email I received. We'll see how it goes.
(Incidentally, it's kind of interesting to see the number of different interfaces that email marketing companies have created for unsubscribing from an email list. There wasn't a common interface among the 15-20 lists that I unsubscribed from.)