keep it brief. Sometimes, one paragraph is sufficient, but make it no longer than 1 A4
type the letter, write it readable anyway. Mind the grammar and spelling and let it read to others, before you submit the letter
always add your name, address, postal code, residence and telephone number. don't be afraid that the editorial office publishes your name unasked. Eventually, mention in your letter that you don't wish your name to be published in the newspaper. The editorial office wants to check if a letter is really coming from you.
You may (internet source added) always enclose pieces of Animal Freedom, make a print from your browser.
Know to whom you address your letter, it makes a difference if the receiving institute takes your letter for notification, or is obliged to handle your letter in public.
raise your credibility by making use of features of yourself, for example:"as a pupil, as a mother, as a dietician, as an activist", etc..
try to write something new to the reader, for example how life looks like for a chicken in the bio-industry, and call readers to do or to give up on something, like buying biological products or signing a petition.
Keep personal complaints to recognizable persons, and scolding, out of the letter, they make you less credible.
Don't repeat the arguments of your opponents, but write about your case in a positive way, for example: "vegetarians get a discount of their life insurance, because they live healthier"
Avoid to denote your opponents as "insensible sadists, lunatics, without a heart, without wit", etc.. Write something like "most people with the heart in the right place, should quit contributing to the passive maltreatment of pigs in the bio-industry".
Don't assume too quickly that the reader understands what you mean, with for example the "bad conditions in the bio-industry", but write about specific things like beak capes, castration without anesthesia, etc..
Positive suggestions are better than negative: instead of "eat no meat", write:"cook a delicious vegetarian meal for once".
Denote animals with "he" and "she" instead of "it".
Write personally and picturesque instead of detached.
Don't use euphemisms like "biology practicum" when you write about "cutting animals"