To be news, a story must be factual, new and interesting. If you want to do PR around an ongoing project or issue that your charity is focused on, you must think of something that just happened to use as a news hook for the release.
The news hook can be within your charity, for example, a fundraising event, or the launch of a new service or campaign; or it could be external to your charity. For example, if controversial research on cancer hits the headlines, you could use that as a news hook for a cancer charity.
A news hook must be of interest to the readership of the publication. For example, local newspapers will only cover stories about things happening in their area; specialist publications, such as the trade press and consumer magazines, will only cover stories on specific topics.
If you want to get publicity in a magazine targeted at a specific demographic, make sure you send them a story that involves or impacts that demographic. For example, if your charity wants publicity in an ethnic minority magazine, such as The Voice, make sure your story involves ethnic minorities.
News becomes old news fast, so if you want to PR something that has happened within your charity, make sure you get a press release out within a few days.