Try to reach out well before, the key is being able to relate to what they do. Do not send the same email to all of them. They should feel like you are giving an exclusive just to them.

Write the reasons why your campaign is relevant to their blog/website/magazine, and how it relates to a trending topic in that environment.

Once you get their attention make them commit to publish the article the morning of the day you are going to launch your campaign. A couple of days before the campaign double check with them, and make sure they remember!

Good luck with your campaign, if you have any further questions feel free to reach out!
Frank