Try
junkbusters.org or look around for "junk mail reduction" etc. You're looking for lists of marketing organizations you can opt-out of. Opt-out of everything you can think of that might sell your info: credit bureaus, credit card companies, your bank, your magazine subscriptions, etc. then write a buch of letters to direct marketing agencies for the same purpose.
Give that a few months, they you are left with individual opt-outs like catalogs. If someone sends you something with a pre-paid return envelope, send it back with a polite removal request.
There's some overhead in doing all that, but it's worked well for me.
Question for everyone: for what classes of mail can we use "refused--return to sender" so that the USPS will return it to the sender (and not just throw it out). Will it also work for mailings with those non-profit stamps?