It is personalized. -- "I have lived in your district for seven years, raised my family..."
The letter you send represents you, an individual voter, an actual person that has real concerns, comments and contributions. By sending (actually mailing) in a letter, you're saying this is an issue worth time, effort and honest contemplation. The personalized letter reminds them that your's is the voice that has the vote.
It is specific. -- "I am writing to you today about .... "
The letter addresses only one topic at a time (remember the sorting piles...) and states the topic clearly in the first paragraph (remember the scanning....) Grab their attention, make your point and move on. You can always send more letters...
It is short.
By short, we mean one or two pages but not much more, at least as a first letter. Think of the first letter as the beginning of a conversation: Introduce the topic, give your initial comments and see what happens.
It is respectful, even if in disagreement.
Differences of opinion are the lifeblood of good debate and a key component of a healthy Democracy. However, disagreements about topics, policies, etc. should never become disrespectful, personal or offensive. Letters that attack, belittle, mock or are offensive or vulgar are simply thrown in the trash, rarely read and never achieve the original intended goal.
Remember: Issues come and go, people tend to endure. The person you are debating with today may be the ally you need in the future.