Letter-writing is a useful campaign tactic when you are trying to:
Letters may be aimed at decision-makers, opinion formers, local media or members and supporters.
For most campaigns in most situations a larger number of letters will clearly demonstrate to the recipient that you have public opinion on your side.
Postcard campaigns or sending identical letters are best reserved for situations where you can guarantee a large number will be sent, because they don't hold as much weight with politicians as individual letters do.
Where things have gone well, explain the impact of their contribution.
Where things have gone badly, explain the reasons and emphasise that this makes their ongoing support all the more important.
When asking for support on a campaign, explain how much impact will be made as a result. If possible, include a sample letter, briefing or other material that will help them.
Don't expect responses right away, but persist if you do not get an answer within a reasonable period of time.
If you are seeking a meeting, it is worth saying that you will be in touch shortly to discuss possible dates.