Make sure the idea is timely. Where possible, I offer photographs to support my submission. Get good stationary, write good query letters. Be on time for appointments or they might think you'll slip your deadline. Also teaches seminars about the writer's life.
Professionalism goes a long way.
Edit for brevity and convey the essence of your idea in one compelling sentence. I often follow the salutation in a query with the punchy lead for the article I'm proposing, immediately followed by that all-important sentence that sums up the story's point. Use the next paragraph to make a case for why the story fits in a particular department or meets a seasonal need and describe how you'll cover the topic. Thank the editor for the opportunity to contribute, and be done with it. I try to read the publication, and understand its readers.