I took a workshop on this during a prefect residential.
I used to be more aggressive that assertive but now I am more assertive than aggressive. Yet, occasionally, I do fly off the handle.
I used this technique last year when I was telling my house mate to never take food out of my plate. I started with ‘is it okay if we talk’, then proceeded with ‘when it comes to food, I am a person that…’ and then the conversation continued. Finally, she understood, that on no account is it ever okay to take food out my place because I HATE (too say it mildly) when people do that.
That was a hard task because for me when I feel passionate about something, it is easy to become defensive but one has to remember that the other person has feelings.
Now when she does it, she just gets the evil look, that I have mastered so well. I tried the assertive way and now I channel the assertive words through a look. :)