Here are some tips from the Sydney Morning Herald on the best way to make sure your letter to the editor gets published:
Keep it short: To make sure you have maximum impact letters should be no longer than 200 words.
Keep it simple: Try to make one or two clear points. You can't solve all the world's problems in one letter.
Keep it fresh: Don't send the same letter over and over. Newspapers like regular letter writers, but they want the greatest diversity possible on every page so you shouldn't expect to be published more than about twice a month or once a week. Save them for your most compelling thoughts.
Keep it civil: Don't abuse people. However strongly you feel about your point, extreme language and wild analogies are unlikely to make it more effectively. We tend to follow the old rule that the first person to mention the Nazis has lost the argument.
Keep it relevant: Your letter will have a better chance if it refers to a story in the paper, on the website, or a previous letter. Not all letters published do so, but most do.
Keep it open: Disclose any relevant information. If you work for an organisation, belong to a political party (especially at election times), have academic links or any affiliation that may be remotely connected to the subject matter of your letter, please mention it. It may not need to be published, but it helps newspapers judge the letter. Additionally the inclusion of sources to back up what you say is valuable, although also not necessarily used for publication.
Get there early: There is no rigid deadline for readers, but there is for the page editors. The later in the day your letter arrives, the harder it is to get it in the following day's paper. Of course it will still be considered, but after another 24 hours the story may have moved on.