The Five Ws: Always tell people who is going to do what, where, when, and why. If you skip the why, people won't understand, won't care, and won't comply.
Book: Poor Charlie's Almanack
Charlie Munger tells the story of Carl Braun, who built oil refineries on time and on budget.
His rule for all the company's communications was called the five Ws: You had to tell who was going to do what, where, when, and why.
If you wrote a letter or directive in the company telling somebody to do something and you didn't tell him why, you could get fired.
In fact, you would get fired if you did it twice.
When you tell people why, they understand it better, consider it more important, and are more likely to comply.
Even if they don't fully grasp your reason, they are still more likely to follow through.