The two keys to assertiveness
There are two keys to assertiveness: 1. Mention an emotion, and 2. Let it go.
Mention an Emotion
The most important key to assertiveness is that you have to use your emotions as the reason that you want to person to change, rather than using logic. The problem with logic is that it tends to go around in circles and it gets very frustrating. Using your emotions gives people less to argue with. It makes the process more efficient.
Let it Go
The particular technique of assertiveness that I recommend is to tell the person the way their behavior makes you feel, and the behavior you would prefer. If they are defensive, then repeat it. If they are still defensive, then let it go (i.e. be nice to them, accept their excuse, move on in the conversation, validate their emotions, even apologize for your tone if necessary). What’s interesting about assertiveness, is that it doesn’t matter whether or not they agree with you. You will still feel better for getting it off your chest, and they will still change.