The following rules are essential if you want people to take you seriously.

Be yourself
Know your subject
Be interested
Punctuate proudly
Respect the apostrophe
Get great grammar
Spell well
Keep to the point
Read and revise
Sleep on it
Pay attention to detail

Be yourself

Write from the heart or the head or the gut, depending upon the type of writing you are doing. You can let your heart pour passion into a love letter but your head is better for the contents of a business letter and the gut feeling should never be ignored. Never try to imitate somebody else