Focus on one or two points, not twelve. Try to ensure that your letter is intelligible to the average reader, who may lack your specialist knowledge. Keep an eye out for juicy quotes and figures for your letters. The letters pages seem to have a very short 'memory', so you need to get your letters to them as soon as possible. You don't stand much chance of publication if you're writing in response to a letter that appeared two weeks previously.
Terrestrial mail comes a very distant third.
They won't publish it. Make sure to include an address and telephone number in your e-mail. Your letter is more likely to get published if it contains some element of novelty eg. The more letters you have published, the more important this sort of thing probably is. Check your facts.