I came accross this very good emails advices yesterday from yourbestemail.com: 

Strip out as many words as possible. Be ruthless. Attention spans are too short.

Remove all “I think”, “I believe”, “it seems like”. Assert, and don’t hedge.

Mark Twain said, “Substitute ‘damn’ every time you’re inclined to write ‘very’; your editor will delete it, and the writing will be just as it should be.”

Blaise Pascal said, “I have made this letter longer than usual, only because I have not had time to make it shorter.” Try not to write emails over five lines.

Don’t offer pretense, or apologize. Get into it.