Writing to MSPs, officials etc.

As with any writing, be clear, concise and organized. Avoid emotional and aggressive writing.
Write as if the readers are reasonable and potentially supportive persons (which is more often true than some people think!) –and so avoid putting their backs up.

All that may be obvious, but there is one common failing that must be stressed. Too many people write at excessive length, believing that to be helpful. It never is (except when journalists are paid by the word). Imagine yourself to be an MSP or public official with many unrelated things to think about every day. Confronted by piles of documents and by innumerable emails, you start reading one that unnecessarily goes on and on (and on), perhaps repeating in different words what, through waffley length, has already caused you headaches on previous occasions. Compassionate though you are, might you not welcome some easy way out, to the detriment of the writer?
A sniper may be more effective than an un-aimed fusillade. If you have written more than a page, consider whether you can cut back. Must every last detail of your case be included? Would supporting evidence be best presented as an Appendix?
If you still feel the need to fire off a barrage of words, ask yourself if your (understandable) motivation is partly despair and a need for catharsis.

There is further advice on this website on writing complaints and case histories.