I used to be fairly successful at getting letters published in the paper. Around 50% got in. These days I doubt it is one in ten. But the reality is, that’s its very much a numbers game. The more letters you send in, the more likely you are to hit the right note and add that “bit of interest” the editor wants.
After all, letter’s editors don’t like having to write their own letters, a situation that no doubt occurs from time to time, and if you are lucky and hit them on one of those days, your letter is going to get published.
Advice:

• SEND IT IN. It sounds obvious, but people forget that unless they send in a letter there is no chance it will be published.
• Read the letters that are already being published. Get to know the style. Most importantly get to know the length.
• Write in the style. “If it the SUN WHAT won it”, write that way. If it is: “anthropogenic synagolism secreted in juxtaposition to heliotheocracy” … tell me what it means – but write it like they want it!
• Start with a sentence that is interesting. Topical, ideally refers to an article already published by the paper and something that grabs the attention.
• Finish with a flurry. Finish with a call for something.
• Have an opinion. Never write a “on the one hand this, on the other hand that”. you are far more likely to get printed if you sound like a passionate fanatic than if you sound like a boring “sit on the fence”.
• The rule of threes: making three points in a row always sounds good:
• You must read it carefully (out loud is best – better get a friend to read it), spell check, and above all … a third point!!! (see point above)
• Ruthlessly cut down your letter. If in doubt, cut it out. If the argument can do without it, do without it.
• SEND IT IN PROMPTLY. … Sounds obvious, but despite all I have said, it is better to send in a lot of bad letters, than to send in one perfect letter a long time after people stop talking about it.
• Add your name and address. Editors want to know you are bona fides. And your name adds credibility.
• BLAME THE POLITICIANS NOT THE READERS OR THE JOURNALISTS. If you want to have someone dig into your mouth to check your teeth amongst the decay and rotten smells of your last meal, you go to a dentist. Dentists don’t complain that it’s an awful job – that’s what they get paid for. Likewise, politicians are paid to take the flak from the public on all types of issues, indeed, they need that public attention the flak gives them, so in a real sense you are doing them a favour!!! So, if you have to blame anyone don’t blame the journalists, because it stands much less chance of getting published, and DON’T BLAME THE READERS. You may as well write “NOT FOR PUBLICATION” at the top. When you blame someone, remember “it’s the politicians fault”, is short hand for “it is the fault of society and we want somone (politicians) to do something”.
• Don’t be afraid to add mild humour. Plays on words often go down well.

Libel
It isn’t a good idea to libel anyone because papers don’t like going to court. Even if you say: “criminal climate scientists” … think if you can back it up (Hansen was arrest for third time). Certainly don’t say “Hansen is a criminal”, it may be true, you may certainly believe it to be true, but the paper can’t publish it without checking the facts and consulting their lawyer, so it won’t get published. The safe rule is not to name names. Now even I am wondering if being arrest is the same as being criminal?

ABOVE ALL – SEND IT IN!!

Even if you don’t get published, the editor will see that they have a number of letters on the subject with a certain view and will tend to include one of those letters.

YOUR LETTER MAY NOT GET IN, BUT IT BOOSTS THE CASE FOR A SIMILAR SYMPATHETIC LETTER.

And, write in plain text, no highlights, no capitals, check the length by counting words – a bit too long may be OK, because many will cut it down.