If you have control over when you release a news story, you need to give it some careful thought. The days have gone when you could hide an embarrassing story under a blanket of news about something else (that tactic is always spotted, and makes the bad news even more prominent). However, you can, and should, consider when to release what you hope is a positive story.
If you are staging a "press event", plan it in the late morning to catch the attention of the early evening news bulletins. If the event has a visual element to it, this is even more important. There's no need to give broadcasters more than a few days' warning, since they work to short timescales. Do make sure that you look ahead to any other events that may clash, and take the assembled hacks somewhere else. For example, it would have been madness to stage a product launch in the City of London at the same time as the G8 meeting was taking place. But guess what? Somebody did. You won't remember who it was, since no press turned up.
If you're aiming for coverage in a monthly magazine, you need to deliver the information to them several months in advance. The Christmas issues are created in the summer, so it's no good pitching a new range of tree decorations in October. Weekly journals are finalised a couple of days before publication, but daily papers, radio and TV can squeeze items in at a few hours notice.
It's all down to planning. If you're too late with your news, it may never appear.