Keep your word if you possibly can. If you have to break a promise, let your friend know. Most people are understanding enough to know that we can't always do what we say. Another requirement for good friendship is time spent together. Spreading yourself to thin makes you everybody's friend and nobody's friend.
Anger has no place in friendships.
We can say some nasty things when we are heated. Anger is one of the best ways to wipe out a friendship account. If you have a problem with anger, work on it. But, leave it out of your friendships. It is more effective to calmly disagree and discuss a subject than to display anger.