You want to stop a patent from being granted in error? Finding prior art is just step one. Succinctly describing its significance is step two, and submitting it to the USPTO is step three. While patent examiners can search this site, the best way to ensure they see your discovery is to submit it using the USPTO electronic filing system.
If you've proposed useful prior art at Ask Patents, there's a good chance a USPTO patent examiner will find that prior art when he or she examines the application in question.
However, to be sure that your prior art counts, submit it to the USPTO.
If you’re going to make a submission, make sure it counts. It’s important to only submit art that seems likely to be relevant. (If you’re not sure, ask a new question about the relevance here on Ask Patents.) The goal is to make a meaningful contribution, not spam a patent examiner’s inbox. You can only submit 3 examples of prior art per patent application before the USPTO charges you a fee.
This Meta post has step-by-step instructions for how to submit prior art to the Patent Office.