ow do you inform someone you're angry with him? E.g. because he didn't do something you expected him to do?

Well, you could try it by putting your judgement on paper, or by using profanity (such as "in stead of sitting on your lazy ass, …" or "you lying moron, you promised to do … ").

Surprise, surprise, … research has shown that such behavior is not really successful. It offends the receiver of the message and often the whole case escalates and gets out of control. Other alternatives with the same effect are known as character assassination: "While you have a constipation of the brain, …" and as use of absolutes: "Can you never do something right?"

This kind of communication of often seen in internet newsgroups (the so called "flame wars), but (as far as I know) it has been studied over the last 30 years. One of the reasons communication can escalate, is because written communication is severely limited (no non-verbal communication), so watch out for what you write.

What can you do about it? Several authors present similar solutions. The book I studied before writing down the solution below is "People Skills", written by Robert Bolton, Ph.D. in 1979, but I've seen other versions (e.g. in Visionary Leadership skills, by Robert Dilts). My solution combines elements from what I've read in these different sources.

What you do is you carefully present your message by splitting it out in 3 parts:

what are the facts: what happened specifically?
what is the effect on you ? (the factual result + your emotional reaction -> how you feel because of it)
what you would do in stead / what you plan to do against it (again: facts, facts, facts). Watch out for giving too much advise, it could be interpreted as an insult to the persons intelligence.
Where did the inspiration from this case come from? Well, I just wrote a letter according these principles to a person living in my neighborhood. He used to burn his garbage on a piece of land not too far from my door, and I was fed up with the smell of it. In stead of starting to insult "the son of a b..", I cited what I saw, what my feelings about it where and I asked him to put his garbage where it belongs. If he wouldn't stop, I told him I'd inform the authorities about the problem. As a result, he came to apologize. My letter had made him understand what were the effects for others. He felt embarrassed he had caused this kind of trouble and he promised not to do it again.