Experts in the art of communication say that the way you convey a message is important at least as the message itself.
Common mistakes to avoid are:
1- Being long winded and repeating the same concept/opinion over and over: this bores the reader and averts his/her attention.
2- Getting personal or hurting personal feelings: this elicits a defensive response and reduces the efficiency of the communication almost to zero.
There would be more things to say, but then I would become long winded...