A Kuro5hin writer has posted a great step-by-step for ensuring that you never receive a pre-approved credit-card solicitation again.
Fortunately, hidden away in the fine print of every single pre-approved offer sent to consumers is a paragraph stating how to prevent credit bureaus from including you in pre-screened lists. If you're like me and always end up throwing these offers away, I urge you to follow one of these procedures to notify the four credit reporting agencies of your request to opt out.