I have the same complaint. Whenever I donate money I always tell them not to put me on any mailing lists or send me any literature. When I worked at Lotus, they had a list of 700 organizations to which you could make donations through payroll deductions. Aside from the benefits of not missing a few dollars from each paycheck and the fact that Lotus matched the donations, I always said one of the biggest benefits was that the donations were essentially anonymous, so I never received any mail from any organizations.

If you get totally fed up, there are donation clearinghouses that allow for anonymous donations. A couple I know of are JustGive.org and NetworkForGood.org (aka Helping.org). The downside is they take a small percentage of your donation for administrative fees (which may be less than what the organization would spend on pestering you).