I would suggest that a polite, non-form letter is a lot more persuasive. Form letters are for those who are either not very erudite or have a problem conveying their polite intent.
In any event, form letters are often just discarded without being read. But more personal letters get a lot more attention. State legislators are often pretty ordinary people and typically don’t get a tremendous amount of polite correspondence from individuals.
Things they look for are an indication of how long you have lived there, since this directly translates to voting friends; that you have some emotional content with this issue, so you mean it; and that you see them as supporting what you (the legislator) supports.